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Good-willed Catholics currently exist in many groups. Below are the arguments that WOULD JUSTIFY a Catholic of good will attending any of these groups. There are other arguments, and I don't have all day, but I'm giving a quick sample so you know what I mean:

Conservative Novus Ordo: "We must stay within the authority and framework of the Catholic Church. Obedience is important. Christ promised perpetual successors to St. Peter. I do my best to do and believe everything a Catholic should. I obey my priest, bishop, and Pope unless they are clearly in error, which some of them are at times."

FSSP: "What Conservative Novus Ordo said, but also the New Mass is defective and dangerous at least. We will play it safe and only say the Tridentine Mass and use the Rites that were used before Vatican II. Vatican II was extremely problematic at best. We will train our priests in separate seminaries so they get a fully Traditional or pre-Vatican II formation. We're OK with the new Rites of consecration and ordination, however."

SSPX: "What FSSP said."  <----- Note how useless the SSPX is now, but I digress!

Resistance: "Vatican II is heretical and destructive of souls, and we have 50 years of evidence to prove it. We should stay away from the Modernist contagion, lest we ourselves become infected. It is permitted to disobey a Pope when he steps outside his authority. God expects us to save our souls, and we need the Sacraments to do so. We have the right to cling to the Catholic Faith as it was always taught. We don't need the Pope or anyone else to give us permission to stay Catholic. But we can keep the Faith and pre-Vatican II religion without denying the papacy of the current Pope. Besides, the Pope has been validly elected and universally accepted by the Catholic Church. Also, the idea of a 60 year interregnum (period between popes) is ludicrous."

Sedevacantists: "What Resistance said, except the stuff about the Pope. The Catholic Church can't promulgate a Mass noxious to souls. And what's the point of Our Lord's promise to St. Peter if his successor could actually be harmful to, and dismantle, the Catholic Church? The last several Popes aren't even Catholic, therefore they can't be heads of the Catholic Church. They are heretics. We know heresy when we see it."

Home Aloners: "I stopped going to Mass after 1970 when the Catholic Church embraced Modernism and error. Yes I've heard about so-called "Traditional Catholics" here and there, but they have no authority from the Pope to operate independent churches. They have no authority to say Mass, much less hear confessions or witness marriages. These groups are like cut-off branches not connected to the main tree, which means they are dead. You can't get life (grace) from dead branches. So I stay at home with my Rosary, and live like the Japanese "hidden Christians" praying for God to end this chastisement."

I don’t believe the “official line” of the fssp is that the new mass is defective and dangerous.  I’m sure there’s some overlap among priests, but I suspect that’s a key difference between the groups.  Sspx thinks it’s “defective and dangerous”.  Fssp thinks it’s just not ideal.

Both R&R and Sedevacantism/Sedeprivationism have problems.

However I prefer R&R because it has some semblance of prior application whereas Sedevacantism/Sedeprivationism have only been theoretical until this point in time i.e for 2000 years. I just don't feel spiritually comfortable with that. But I do not think Sedevacantists/Sedeprivationists are schismatics or heretics either.

Yes, one of the questions to consider carefully, on which the whole future of the traditional movement depends, is (1) Is the New Mass invalid, like a Protestant service? (2) Is it blasphemous, even if valid, like a Black Mass?, or is it (3) the New Mass is only inferior, like a truncated Mass would be? The Truth is certainly in Tradition, and one of these positions must certainly be true, but there is debate among us as to which it is. I would argue for (3). Latin Mass Society has a comparison of the texts here https://lms.org.uk/missals

One of the most problematic aspects of the New Mass is the watered-down, ambiguous Offertory Prayers. A beautiful prayer like "P: Receive, O Holy Father, almighty and eternal God, this spotless host, which I, Thine unworthy servant, offer unto Thee, my living and true God, for my countless sins, trespasses, and omissions; likewise for all here present, and for all faithful Christians, whether living or dead, that it may avail both me and them to salvation, unto life everlasting. Amen." got replaced by a watered-down, "P: Blessed are you, Lord God of all creation, for through your goodness we have received the bread we offer you: fruit of the earth and work of human hands, it will become for us the bread of life.", which can be taken in a Catholic sense, but also in a heterodox sense. 

In spite of this and other manifest problems in the New Mass, most who have examined it conclude it is valid, and contains no heresy. That conclusion also follows from observing that the only words changed in the Words of Consecration are "Mysterium Fidei", the Mystery of Faith. Now, that removal will surely lessen reverence, especially in the Priest, but it will not invalidate the Sacrament. Fr. Garrigou Lagrange, writing before the Second Vatican Council, said those words are not essential to validity. One of Father's theological proofs of the same are that those words are not found in Eastern Liturgies like those of the Greek Church that are valid.

Now, if it is valid, and contains no heresy, as even Bp. Williamson has argued recently, then it confers grace ex opere operato. I would say a valid Mass, celebrated in the Catholic Church, always obtains at least some graces, and a valid Communion, distributed in the Catholic Church, always confers at least some graces on those who receive them properly disposed (in a State of Grace). Let's say it obtains 5% of the graces obtained by the Traditional Latin Mass, which is the Complete, Full and Integral Mass. A truncated Mass with some prayers changed arguably does not have no grace at all, but neither does it contain the plenitude of grace as the true Mass does.

Of course, with regard to Holy Communion, another aspect must be considered. The traditional Mass has so many safeguards against sacrilegious Communions also, while in the New Mass, especially with the abolition of many important prayers, without Altar rails, and with the lax and sinful distribution of Communion in the hand etc, sacrileges have multiplied. That also has to be taken into account. 

Ladislaus, about Eucharistic Miracles etc, I do not use it as a standalone matter or as the only definitive proof. I use it, as also Bp. Williamson does, as part of a cuмulative case. It constitutes at least significant evidence that must be taken into account, however you wish to argue about it. Scientists have said Eucharistic Miracles cannot so easily be faked: https://dowym.com/voices/5-incredible-eucharistic-miracles-from-the-last-25-years/ Likewise, with Dr. Marshall's or other conversions etc; sure, that's not an absolute proof, and other explanations, like those you gave, are possible. It is part of the ongoing examination/study on the fruits of the New Mass that must be taken into account. I believe if the New Mass is celebrated reverently, versus Deum, with the Roman Canon (Eucharistic Prayer I in the New Mass), with Altar Rails, Communion kneeling and only on the tongue etc, at least 20% of the Graces of the TLM will return.

Yes, we agree there have been horrible loss of graces in the last 50 years, and no doubt the Mass is at the centre of that. But it can also be explained by an inferior Mass, imo, and not only by an invalid Mass, or one blasphemous/ heretical etc. One final thing. It is said that when Anti-Christ finally comes, the Mass will be abolished for about 3 1/2 years. And that will be the worst crisis of all. Doesn't that itself suggest that, at no point before then, the Mass will be completely abolished? Bp. Williamson said graces are "strangled" at the NM.

God Bless.