If such a thing were possible, it would, at the very least, reduce the papal election laws, ceremonies and procedures, both before and after the election, which laws btw, have themselves been established by popes, to nothing more than ceremonies used as instruments designed to deceive the whole world - if such a thing as electing a usurper to the Seat of Peter were even possible.

I hold the belief, as most theologians do, that the Pope cannot ever become a heretic,

I do believe that no usurper was ever elected.

If however, Siri was actually elected and deposed (there is no method within the Church at all to depose a pope, canonical or uncanonical), Siri himself would in fact be guilty of at least a grave sin of omission by his denial or silence in the matter, which under those terms, he would be guilty of doing what sedes (wrongfully) say popes cannot do - sinning via the breaking the Church's "universal law" (which it really isn't) of papal elections, thus ipso facto he vacates the office anyway. So why believe in that conspiracy theory?

Wrong.  Siri stated that grave things happened at the conclaves but that he could not speak about them due to the secrecy oath.  And there's a lot of evidence (albeit no smoking gun proof) that he was in fact elected.

Reality proves that not even manifest heresy or apostasy has caused a pope to vacate the office.

What reality are you looking at?  I see a Concilar Church that doesn't resemble the Catholic Church of old.  That's the reality I'm confronting.

We would not expect one who does not even defend his own election to the papacy to defend the faith, quite the opposite. A man such as that is expected to defend the faith just the same as he defended his election, iow, just the same or, likely even worse that the conciliar popes, i.e. not at all.