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The Universal Acceptance has only fully come to light more recently

This is not some recent development.  UA simply does not apply to this situation.

You try to pretend that S&S came up with some amazing new discovery in putting together a badly-flawed chapter on UA.

Right, +Williamson never has considered their legitimacy to be dogmatic fact ... nor did Archbishop Lefebvre.

Well, just for clarity, based on my phone conversation with Bishop Williamson on the subject, he appears not to have studied the matter.

So while his conclusion (ie., the technical possibility of sedevacantism) would logically suggest he had rejected the claims of universally accepted pope’s being dogmatic facts, it appears he reached the conclusion of the technical possibility of sedevacantism in reliance upon Archbishop Lefebvre’s occasional words that such was a possibility, and not because he thought the UA argument didn’t apply.

What would BW conclude if he refreshed his studies on the matter?

I dare not speculate.

But it would seem the real question should center around +Lefebvre (since +Williamson’s opinion is formed on reliance upon +Lefebvre’s private tolerance and occasional opinion regarding its possibility).

Interestingly, +Lefebvre did sometimes raise the universal consent argument (cited elsewhere), and at other times, make statements suggesting the sede possibility.

Were these flare-ups he later regretted, rather than actual changes of position (ie., words said in the heat of battle on occasions of grave scandal)?

That would seem reasonable, yet Bishop Williamson does not suggest that as an explanation.

I would like to converse with him more deeply on this point, to see if +Lefebvre’s tolerance was more sporadic/occasion-based, or sustained.