That said, if a stranger showed up to my door out of the blue, and claimed the Virgin Mary had sent them to rush me off to be consecrated, you would be fully justified in questioning my mental capacity if my response was, “Let me pack my bag!”

No, the tutiorist approach does not require scruples.  It's easy to confound this with scruples.

Bottom line is that there's no positive doubt regarding most of the Thuc consecrations ... not the ones that can be traced clearly back to Thuc ... e.g. the des Laurier line (McKenna, Sanborn, etc.) nor the Carmona/Zamora line.  Some of them are shady ... with little proof they actually happened.  Anyone can set up shop and claim they derive from the Thuc line.

The standard or threshold for the mental competence to validly confer a Sacrament is very low.  You basically just have to know that you are a bishop doing this thing the Church does to make bishops/priests.  Unless Thuc were walking around with drool coming out of the corner of his mouth barely answering to his name ... the presumption is what he was mentally capable.  People who knew Thuc attested to his mental sharpness ... relating stories that he could switch seemlessly between modern languages in having conversations with groups of priests.

Having done something irrational ... such as the Palmar consecrations ... absolutely does NOT rise to the standard of establishing mental incompetence.  It was actually an SSPX priest who went to +Lefebvre first to ask him to do the ordinations/consecrations.  +Lefebvre was the one who then referred the priest to +Thuc.  So Thuc was persuaded to go, and he was evidently taken in by the preternatural phenomena on display at Palmar, and was persuaded that Our Lady wished the ordinations and consecrations.  Having been gullible and suggestible doesn't mean you can't validly confer a Sacrament.  Recall that this was BEFORE Clemente declared himself pope.  As soon as that happened, Thuc renounced the group and broke all ties with them.

So it's mainly if not exclusively the SSPV that created all this FUD regarding Thuc.  It is reported that Bishop Kelly made a statement to the effect that, "We can't tell people they're valid because then they might go to them."  Doesn't sound like an unbiased source.

NOW, ironically, the +Mendez consecrations DO rise to the level of creating doubt.  +Mendez had been hospitalized for a stroke very shortly before he did the consecrations, and family members who visited him in the hospital after that declared the +Mendez didn't recognize them.  So what kind of state was +Mendez in when he did the consecrations?  Stroke could in fact result in a mental impairment sufficient to invalidate the Sacraments.  I find it ironic that SSPVers have no problem going to Masses offered by Fathers Greenwell and Baumberger (Father Zapp relates that Mendez appeared to have almost deliberately garbled the pronunciation of the essential form and after the assisting priests told him to repeat it twice, the answer to Father Kelly's "Did he get it right that time?" was an "I think so.")  So they are OK with that but then create all kinds of stink about Thuc, whose issues were much less profound.

No one has been able to demonstrate that Thuc was mentally impaired.  Period.  Consequently, the validity of the Sacraments conferred by him is presumed.  No need for "tutiorism"  You could apply tutiorism to avoid priests ordained by +Lefebvre because of the ridiculous Tisserant-mason allegations.  But the questions is whether the allegations are real and well founded, and rise to the level of positive doubt.  They do not in the Tisserant allegation, nor do they in the allegations against Thuc.

I think you need to read this:
http://archives.sspx.org/archbishop_lefebvre/validity_of_holy_orders.htm

.
I've read it and agree with it.  I was essentially agreeing with Ladislaus, that the criteria used to evaluate Thuc would seem to render ABL's orders equally doubtful, and I was pointing out to you that the fact that there were co-consecrators accompanying Leinart is totally irrelevant since if Leinart's intention is doubtful then Lefebvre was never a priest in the first place, and you can't be made a bishop unless you're a priest first.

testimony regarding his mental state by others at the time

What does that term "shadow of uncertainty" even mean?  That sounds almost like the very definition of negative doubt.

Were it not for the antics and propaganda of the SSPV, no such uncertainty would exist.

Let me ask, do you consider the +Mendez ordinations/consecrations to be similarly doubtful?