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And even if someone was aware of the Crisis but truly believed the Novus Ordo rite was valid and proper, and held the Catholic faith in whole,

This is a contradiction.  What someone believes is different from the Truth.  A Novus Ordo catholic cannot have the "whole" faith, because the novus ordo is not wholly catholic.

I don't think they're a heretic or a schismatic or the like. In error and needing correction? From our view, and we should endeavour to do that.

If someone is in error on the Faith, (and we're talking about substantial error), then by definition, they are either a material heretic or a schismatic, or some combination thereof.  And yes, I agree, it's our duty to teach them, if they will listen. 

But some heretic or schismatic damned to Hell, as the Dimonds and co. would tell us? Hardly.

It's not our job to damn anyone to hell.  It's not even the CHURCH's job to do that.  And the Church has NEVER done so, on an individual basis.  But...the Church does teach that ignorance of the Truth or a holding to an error (even if the error is held "sincerely") will not suffice for salvation.  How this applies to Joe Doe is up to God.  But we can't water down Catholic Truths and we cannot say that someone who objectively holds error (even part error) has "the Faith".  This is lowering the bar for everyone.
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We must pray and work tirelessly to convert all those who are not 100% orthodox, even if they think they are.  Christ died on the Cross for all men, but only some will accept His grace.  For those that will, many are not yet part of His Church, or are only partly.  As He told us in Scripture: "And other sheep I have, that are not of this fold: them also I must bring..." (John 10:16).  It's our job to bring these others to the fullness of the truth and that starts by recognizing their errors so we can instruct the ignorant.

Well, let's pray that he receives the grace of conversion before death.

I'M KIDDING! :)

It's a good example of what I mean though.

I personally consider the FSSP to be overall a very compromised group. But the individuals in the group are obviously Catholic! They are very conservative Catholics, certainly in possession of the Catholic Faith.

About the FSSP:

Traditional Catholics, from the very beginning, have held 3 essential principles:
1. Leave the Conciliar Church and Novus Ordo behind completely, regardless of having a Trad option lined up for Mass. Stay home on Sunday if no Tridentine Mass available.
2. Seek out SURE sacraments in "lifeboats": valid priests to offer both the Tridentine Mass and traditional Sacraments, and support them wherever they are, even at independently set up "Traditional chapels" which Rome considers "illicit" or "un-canonical". Treat them for all practical purposes like one's local parish.
3. Believe "I don't need permission from the Pope, or Modernist Rome, to stay Catholic". That is, believe in supplied jurisdiction for Mass and ALL sacraments, and have NO scruples about attaching oneself to, and fully supporting, such Trad chapels and building them up.

Those elements are common to 100% of Traditional Catholics. Anyone who scruples about going to Confession to a Trad priest because he doesn't have official jurisdiction or faculties from the local Bishop, for example, is not a Trad. He might be conservative, but if he doesn't understand that Catholics have a right to keep their Faith, and a need for the Sacraments, they have a lot to learn and therefore are not yet "Traditional Catholic".

Enter the FSSP:
Since part of their essence is "approval/permission from Rome" they are technically not Trad, since such approval was not part of the Trad package from the beginning. But more importantly, it actually contradicts principle #3, above. From the beginning of the movement in the 1970's, getting permission from Rome was certainly NOT one of the elements of the Traditional movement. So the FSSP is a new kind of conservative Catholic at best ("Indult Catholic"? "Latin Mass Catholic"?).

While Sedevacantists ALSO added another element to the Traditional manifesto, "4. The putative pope is not pope at all.", this is different in two main ways:
1. A good number of Trads believed this from the beginning. It was there, just not universal. So they are a specific sub-type of Trad, rather than SOMETHING OTHER THAN Trad.
2. Their additional belief doesn't contradict Trad principles 1-3.

This raises a couple of questions from me."

1: I get the reasoning behind staying home rather than attending a Novus Ordo, but would you also stay home rather than attending a Tridentine Mass at a place like the FSSP if those were your only options?  And if so why?  Why would doing that increase your chances of saving your soul?

2: What if your only option was a Byzantine Rite liturgy that hasn't been changed with Vatican II, but is still in communion with what you'd call the Conciliar Church?  Would you stay home rather than attending and receiving the sacraments there?  If so, why, and why do you think that would increase your chances of saving your soul?