Suppose a woman has a miscarriage, and is advised by her doctor that, if she and her spouse abstains from the marital act for 90 days, it will improve the chances of procreating a live birth.

In this case NFP/abstinence is used not to frustrate or avoid childbearing, but in order to INCREASE THE CHANCES of a successful birth.

Yet if NFP/abstinence is intrinsically evil, this advice could not be followed, and the primary end of marriage would be frustrated.

It is clear, therefore, that NFP/abstinence is not intrinsically evil.

Circuмstances can make it good (promoting the primary end of marriage).

That is to say: Whether one calls it NFP or abstinence, the goal of temporarily avoiding procreation is not always evil.

It is not surprising to me that the Feeneyites here have a problem understanding this, since just as in the case of baptism of desire, what they are really doing is reacting against the potential for abusing the principal, and that reaction leads them in both cases to overreact and condemn the principle itself.

You are conflating too many things.  You conflate abstinence with NFP.  Abstinence on its own is not intrinsically evil.  Nobody has ever said that it is.  NFP already entails, by its very definition, the intent that we are discussing.  FORMAL INTENT, which is extrinsic to the abstinence, can indeed make it good or bad.  Abstaining in order to do penance for one's children ... that can render it good.  Abstaining for 90 days, per your example, when done for any just, by mutual consent of the spouses, has no sinful implications.  So I have no idea what you're going on about.  You're setting up a straw man for a position that nobody has ever argued in favor of.

No doubt, anyone who reads this thread who is not a long-time consumer of Cathinfo will likely be alarmed and confused at the state of this thread.  As a PSA, let me summarize the two arguments that are being pitted against each other.  My goal is to clarify and defuse, since things have so immediately descended into strawmen and ad hominens.  I am not offering my own argument in its most developed sense so much as I am simply summarizing two competing arguments to the benefit of the reader, I hope:
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Ladislaus's basic position is that periodic continence is always wrong in practice because it necessarily includes a formal motive of "not intending to conceive" while still pursuing the marital act.  This motive, he argues, (and has argued for some time) is contrary to the teachings of Pope Pius XI who taught that in order for marital relations to be lawful, the secondary ends of the act (the allaying of concupiscence, spousal bonding, etc.) must be subordinated to the primary end (procreation).  He maintains that the encyclical Casti Connubii establishes two distinct principles, the one which condemns contraception, the other which condemns the use of periodic continence.  Here is the paragraph on which he bases that view:
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[Sterile relations are intrinsically lawful because] in matrimony as well as in the use of the matrimonial rights there are also secondary ends, such as mutual aid, the cultivating of mutual love, and the quieting of concupiscence which husband and wife are not forbidden to consider so long as they are subordinated to the primary end and so long as the intrinsic nature of the act is preserved. (§59, Pieran Press Translation)

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So, sterile relations are lawful so long as the intrinsic nature of the act is preserved (i.e., so long as contraception is not used) and so long as the ends are properly ordered (i.e., so long as your motive of intending to have children is mentally prioritized over your motive to gain any of the other rewards of marriage).  One can see how the text implies a distinction between these two things. Periodic continence does not include a disruption of the intrinsic nature of the act but, by definition, it does mean subordinating the primary end to the secondary ends because when one uses periodic continence one withdraws the intention to procreate.  Ergo, it is condemned.
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My position is different in a subtle but highly significant way.  My position is that in order for marital relations to be lawful the intrinsic nature of the act must be preserved, and that if it is preserved, then the ends are duly ordered.  I believe that although the quote Ladislaus uses to support his position does seem to read the way he reads it, it is an inferior translation and it does-- objectively, manifestly, and matter-of-factly-- differ from the preconciliar translation, as we shall see in a minute.  
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I do not believe that Castii Connubii, in the relevant section, has anything at all to say about motives, intentions, etc.-- it is concerned completely and entirely with the nature of the act itself rather than with the nature of the and the motives for acting.  My position is based in what I believe to be the true meaning of Casti Connubii based on:
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1) It's Latin original
2) It's pre-conciliar Denzinger translation
3) The lengthy explanations of it from its drafter, Fr. Arthur Vermeersch.
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Here is the translation I am using-- it is the preconciliar, Denzinger translation (by DeFarrari).  Notice how it differs in a subtle but significant way from what Ladislaus is relying on:
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[Sterile relations are intrinsically lawful because] in matrimony itself, as in the practice of the conjugal right, secondary ends are also considered, such as mutual aid, the cultivation of mutual love, and the quieting of concupiscence, which spouses are by no means forbidden to attempt, provided the intrinsic nature of that act is preserved, and so its due ordering is towards its primary end. (Casti Connubii §59, Denzinger/Defarrari translation §2241)

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There is only one principle being elucidated here: Pius XI says that such relations are lawful because so long as the intrinsic nature of the act is preserved, then the ends of the act are duly ordered toward the primary end.  It's basically a conditional: if the act is preserved, then the ends are duly ordered.  I would maintain this is a more accurate reading anyways, since the theological context is a discussion of intrinsic ends (i.e., those which pertain to the nature of acts) rather than extrinsic ones (which pertain to the reasons or motives for which one acts).  I think that without the help of an inferior translation, one would never naturally read this paragraph to be describing motivations, intentions, etc.
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For context, the Latin that is being translated is "...dummodo salva semper sit intrinseca illius actus natura ideoque eius ad primarium finem debita ordinatio." (AAS 22, 539). 
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I would maintain that the Latin does not imply the kind of distinction that might be read in the translation on which Ladislaus is basing his argument.  As usual, the traditional translation is the more faithful one.  That is my position. But, not being a particularly capable Latinist, I suppose it is possible that I am wrong (although that would also mean the traditional Denzinger translation is wrong, and that we had to wait about sixty years to get an accurate one).  However, the man who ghost-wrote Casti Connubii, Pius XI's chief moral theologian Fr. Arthur Vermeersch, understands it in exactly the same way:
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As long as the act takes place normally it remains objectively directed toward its primary end, which is generation; and since, according to the maxim that the purpose of the law is not within the matter of the law (finis legis non cadit sub legem), there is no obligation, while observing the law, to intend the end for which it was promulgated, it follows that the act is not necessarily vitiated by deliberately choosing a certain time with the intention of avoiding conception. (What is Marriage? A Catechism arranged According to the Encyclical Casti Connubii, 1932, p. 44, emphasis added)

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I am satisfied that the combination of the original translation and text, combined with the expert explanations of the man who was closest to the drafting of the docuмent in question, are sufficient evidence to support my position: periodic continence is intrinsically lawful on account of the fact that, since it preserves the intrinsic nature of the marital act, it maintains the due ordering of secondary to primary ends.  If I am wrong, I am not "obviously" wrong, nor can the broad polemic that Ladislaus has initiated against me-- describing me as a public sinner who promotes public sin, etc.-- be maintained without throwing far greater men than I under the bus of public sin, stupidity, malice, etc.  
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The furthest Ladislaus has gotten in responding to my argument that Casti Connubii doesn't say what he thinks it does is by accusing my position of regarding intentions as irrelevant to morality.  For one, I've explicitly admitted on numerous occasions that intentions absolutely do factor into whether an act is moral, and I have explicitly warned against the use of PC for insufficient motives, and offered my own opinion that doing so would likely constitute a grave sin against marriage.  So it just isn't accurate to say that my position requires one to jettison the role that motives play in human morality.  But there is a second and more important problem.  And that is this: the whole point of dispute is whether or not a withdrawal of the intention to conceive constitutes the undue ordering of ends in the first place.  Ladislaus says that it does, and the current status of the question is that I have argued it doesn't because the encyclical treats the ends as duly ordered so long as the nature of the act is preserved.  So in accusing me of ignoring motives, Ladislaus simply begs the question.  The exchange between us has been so incommensurate that if I get a reply, it will likely be an isolated quote (maybe the last sentence even, where I claim he begs the question) followed by a Lutheran-style-polemic that twists my position into a strawman and then makes an ad absurdum reduction from it.  But this really is the state of the question: does Casti Connubii tell us what Ladislaus says it does?  I say it doesn't, and I've given my reasons.  Now, Ladislaus is of course "free" to continue maintaining his position, but he is not free to insist that it is based on Pius XI's teaching.  He can keep making the same argument based on his own intrinsic reasoning (which is really all that he's been doing this whole time anyways), but he can't claim support that it's a position supported-- never mind obviously or in black and white, as he likes to say-- by Casti Connubii.  He's free to make a different argument too, of course.  Though one would hope-- given the emotional currency he has tied up in the present argument-- that he would first acknowledge the present state of the question so that we could have a moment of intellectual catharsis.

  My position is that in order for marital relations to be lawful the intrinsic nature of the act must be preserved, and that if it is preserved, then the ends are duly ordered.

Mith, you are correct, but only in the sense that the ACT is lawful and duly ordered.  Where you are incorrect, is that the INTENTION to avoid conception is NOT LAWFUL, as the sacrament of matrimony has as its purpose the God-ordered duty to "increase and multiply". 
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Periodic abstinence is only allowed for grave reasons, which in pre-V2 days required the permission of a priest.  Those who practice it without permission and without a grave reason, are wrong.

The best booklet on NFP is:  Natural Family Planning and the Christian Moral Code.

Get it from https://fatherwathen.com/product/natural-family-planning-and-the-christian-moral-code-by-jeanne-dvorak/

In 1940, Archbishop John Murray of St. Paul did not hesitate to oppose and condemn – publicly – what he called the “notorious and malodorous Rhythm System gaining publicity out of Chicago."  He forbade his priests to preach it or teach it, and bookstores from selling literature promoting it.    Clearly it can be seen from this that despite any mistaken, so-called “approval from Rome,” NFP was both new and evil – a veritable nuclear bomb on the Catholic scene.  Neither customary nor traditional, it created the highest controversy, and rightly so.  It broke the ground for the future flood of birth control.    




 

Given the facts, I would agree that the Church infallibly teaches that periodic continence can be lawful to use.

1.  Has nothing to do with infallibility.
2.  Periodic continence is NOT the same as NFP.  Not even in the same ballpark.
3.  "Can be lawful" means with priestly permission and under grave circuмstances. 
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The alternative is to suppose that a hundred years worth of popes and theologians universally erred on the matter.

100 years out of 2,000 is now a "traditional" teaching?  That's hilarious.  You admitted that over this 100 year period, there's not a consensus on the "upstream principles and downstream applications".  That doesn't sound like a consensus in any degree.  It sounds like a highly debated topic. 

The knowledge: anatomy and physiology always was.  When Dr. Billings found info, it was from a book that was dated at the turn of 1900, stating "if you wish to become pregnant, watch for the cervical mucus.  And Indians knew of it as well.  So, it always there.  Sin.  Sin is in the heart, attitude.  Like intent.  Our hearts will be judged.

The next time the issue was raised was in 1880, when the Sacred Penitentiary issued a more general response . The precise question posed was this: “Whether it is licit to make use of marriage only on those days when it is more difficult for conception to occur?” The response was: “Spouses using the aforesaid method are not to be disturbed; and a confessor may, with due caution, suggest this proposal to spouses, if his other attempts to lead them away from the detestable crime of onanism have proved fruitless.” (This decision was published in Nouvelle Revue Théologique 13 [1881]: 459–460 and in Analecta Iuris Pontificii 22 [1883], 249.)

Right, we have gone over this before.

"MORE DIFFICULT" is different than impossible ... this constitutes a venial sin (vs. impossibility ... NFP when carried out is as effective as birth control).

IF AND ONLY IF OTHER ATTEMPTS to lead them away from contraceptive practices have failed, THEN it is permitted WITH DUE CAUTION (without actively condoning the practice) for them to be LEFT ALONE (i.e. tolerate the venial sin if the Confessor judges that their practice of mortal sin is otherwise inevitable).

Absolutely none of this is an endorsement of NFP.  If it is not sinful, then there's absolutely no reason for all the caveats and conditions ... the if/and only if, the "with due caution", "not to be disturbed" ... as opposed to being openly told it's OK.