The heart of the matter in all the controversy over +Williamson's statements to the emotional woman in Mahopac, NY last summer is:

Is the Novus Ordo Mass intrinsically evil like a Black Mass?

Intrinsically evil means "evil in its very essence", an evil you can't possibly escape from due to circuмstances. For example, Abortion, Sodomy, or interfering with nature to prevent conception from occuring. Or any sin, for that matter. You can never sin that some good might come from it, because sin is an infinite offense against God, and intrinsically evil.

A rotten apple isn't intrinsically evil. It's a great food for certain farm animals, or at least the compost pile. Likewise a rusty nail can be sold to the recycling place and melted down to make something useful. A flat tire can be used to make a garden bed or planter. An empty 2-liter bottle can be used to store water. And so forth.

It doesn't matter who you are, where you live, what your circuмstances -- a priest could never permit a person to get out the forceps or saline solution and have an abortion, or commit sodomy, or attend a Black Mass. No matter what your state of mind, subjective opinion, your family's beliefs, your economic situation, etc.

(A woman with life-threatening cancer might undergo chemotherapy which could kill the baby, but this is "double effect" and she isn't willing the death of the baby. She's not "having an abortion", she's undergoing medical treatment, which MIGHT HAPPEN TO kill the baby as an unintended consequence. There's a difference.)

But if the Novus Ordo is truly as evil as a Black Mass, for everyone in all cases, then why don't we all:

A) deny that ANY converts from the Novus Ordo were Catholic
B) require converted satanists/Novus Ordo Catholics to undergo a full catechumen process, formal abjuration of their heresies/satanism, and baptism
C) Require converts from the Novus Ordo to confess how many times they attended the Novus Ordo as part of their "first confession"
D) become sedevacantists (and, like most sedevacantists, become ecclesiavacantists as well), because we're talking about the "Catholic Church" pushing a positively evil BLACK MASS EQUIVALENT on the entire world! The Catholic Church could never do that.
E) get SERIOUSLY vocal (shouting at a minimum, throwing things optional but preferred) with our friends and family members who still attend the Novus Ordo. Forget ignorance, good will, familial relationships, civility, etc.! Would you let a friend go to a Black Mass every Sunday without saying something, and taking serious action? I didn't think so. We should be boycotting them, totally cutting them off, and making a huge deal out of their weekly blasphemous attendance at a Black Mass.

A rotten apple isn't intrinsically evil. It's a great food for certain farm animals, or at least the compost pile. Likewise a rusty nail can be sold to the recycling place and melted down to make something useful. A flat tire can be used to make a garden bed or planter. An empty 2-liter bottle can be used to store water. And so forth.

A rotten apple, rusty nail, etc., are what they are because God created and intended them to be that way.  They are in themselves good.

The Novus Ordo is a creation of man, and is not simply a transmutation of something good from one form into another.  It is an entirely new creation.  It pretends to be what it is not, an altogether pleasing, holy, and acceptable sacrifice of propitiation.  

By their fruits you shall know them.

A) deny that ANY converts from the Novus Ordo were Catholic

If you can demonstrate that their intention is to fall away from the faith and to abandon it, rather than taking part in an evil act because of confusion or genuine ignorance, then perhaps we can say they were not Catholic.  

B) require converted satanists/Novus Ordo Catholics to undergo a full catechumen process, formal abjuration of their heresies/satanism, and baptism

Your extreme lumping of satanists and Novus Ordo Catholics is not a fair approach, and appears to be designed to elicit an emotional, knee jerk response.  The satanist KNOWS and INTENDS to do what he does.  The Novus Ordo Catholic, one presumes, does not have the same level of maliciousness.  

They should, however, undergo full catechetical instruction, confession, and form the intention to amend their lives by not returning to the Novus Ordo.

I fail to see how attendance at the Novus Ordo demands that one be re-baptized, unless you are saying that if one holds the Novus Ordo to be intrinsically evil, it somehow effects the validity of the sacrament of baptism.

C) become sedevacantists (and, like most sedevacantists, become ecclesiavacantists as well), because we're talking about the "Catholic Church" pushing a positively evil BLACK MASS EQUIVALENT on the entire world! The Catholic Church could never do that.

Another extreme "either/or" example. The onus is on you to demonstrate that the "Catholic Church" pushed the Novus Ordo Mass on us, rather than that evil men in the hierarchy abused their authority.

An interesting tidbit is that nowhere does Paul VI command or give any priest permission to use the new Missal in the celebration of Mass.  They behave as if he did, but if you read Paul VI's Missale Romanum, it simply doesn't exist in there.

D) get SERIOUSLY vocal (shouting at a minimum, throwing things optional but preferred) with our friends and family members who still attend the Novus Ordo. Forget ignorance, good will, etc.! Would you let a friend go to a Black Mass every Sunday without saying something, and taking serious action? I didn't think so. We should be boycotting them, totally cutting them off, and making a huge deal out of their weekly blasphemous attendance at a Black Mass.

You need to review the moral theology manual regarding the conditions/qualities for/of fraternal correction.  And also, the extreme "either/or" approach to your question should be pointed out to the board.  

C) Require converts from the Novus Ordo to confess how many times they attended the Novus Ordo as part of their "first confession"

I certainly confessed going to the Novus Ordo as a sin when I made my general confession after I started going to my SSPX chapel.

The intrinsic evilness of the new mass is irrelevant to whether or not one can attend it.  Even if it's not intrinsically evil (it's certainly arguable that it is), one cannot attend because:
1) it's illicit and therefore sinful
2) it's circuмstances (i.e. liturgy) are sacrilegous, therefore sinful.

To judge the morality of something, we must go back to the 3 elements of an act.  If an act fails any of the 3, it's sinful.
1.  The act itself
2.  The circuмstances
3.  The intention of the individual

Even if the act of the novus ordo (#1) is instrinicly NEUTRAL, it is sinful due to the circuмstances (#2), regardless of one's (good) intentions (#3).  

It might be a good idea for all of us to review what the SSPX used to say regarding the New Mass, and stop making excuses for it.  

IF the New Mass can be said in a way that is not intrinsically evil, that is not offensive to God and offensive to pious ears, then priests of tradition should be saying the Novus Ordo that way.  For then there would be no real reason to refuse obedience to the legitimate authorities of the Church, neither is there cause for setting up private chapels outside the diocesan structure.

Unless, of course, you hold the sedevacantist or ecclesiavacantist (hey, a fancy new label!) view.

The Novus Ordo is certainly defective ("evil" in that respect) and was created
by men (not God),
from scratch (an unacceptable rupture with the sacred Tradition of the Apostles, and the organic growth that occurred over 19 centuries)
by protestants
and appears to have many Freemasonic elements, for those who know about Fɾҽҽmαsσɳɾყ

So no, the Novus Ordo is not acceptable for decently intelligent, aware and/or informed Catholics to attend.

The question is, could you be sanguine about a Downs Syndrome sufferer attending a Novus Ordo Mass if he was pious and insisted he was getting grace from it?

The same way you let a 95 year old woman alone if she watches the Mainstream Media and is totally convinced about terrorists and all the other boogeymen, and doesn't believe the truth about 9/11 (that the CIA and Mossad perpetrated the attack as a fαℓѕє fℓαg). What harm would it do, really, to let this poor old woman have her MainStreamMedia (MSM) fantasy? She won't live much longer, she doesn't have any kids who she needs to steer away from the military, etc.

In certain cases, truth or "the ideal" isn't an issue, because some people don't have use of reason, or perhaps they don't stand to profit much from the truth (or suffer much from error) due to extreme age, poverty, or other circuмstances.

Wouldn't the struggle, stress, and effort to convince a 95 year old woman about the truth of 9/11 to be more trouble than it's worth? What are the downsides to this woman continuing to believe an error of this nature? What are the potential gains? Those are the questions we must ask ourselves.

And besides, how do you convince a sincere, pious mental child to stop attending the Novus Ordo? Are you going to give him a copy of "Open Letter to Confused Catholics"? He can't even read. He has the mind of a 6 year old.

No, you can never choose a "doubtful" Liturgy in place of a certain one -- neither in the abstract nor in the concrete.

And St. Pius V declared in Quo Primum that the Tridentine Mass, at any rate, would always be acceptable for priests to say and Catholics to attend.

That is what I call sticking with what we know in a time of crisis. That's the very definition of being a Traditional Catholic.

It's not a matter of choice if the legitimate authorities make it clear that this is the rite they want you to use.  Again, IF the NO can be said in the way I indicated above, THEN the conclusion given above stands.