Maybe. He still fell into heresy by supporting and enforcing the Vatican II sect, therefore, losing any papal authority he may have had.

Would not embracing Vatican II with it's teachings on religious liberty constitute heresy?

Did he embrace them?  Secondly, no, religious liberty as a term is not heretical.  1) it's never been defined with the note of de fide and 2) there's probably a different understanding of the phrase depending on who you ask.  I've seen a lot of people (conservative Novus Ordites) asser that it basically means religious toleration.

I think we just had a round of where I was defending you against the charge of heresy from RomanTheo.  When we asser that someone is a heretic, we'd better have 1) chapter and verse of which dogma they're directly denying and 2) proof that the person denied it.

To charge Siri with heresy, I'd need to see 1) citations from Siri himself and 2) an explanation for why it directly contradicts something that's de fide.  And then if you confronted him about it, would he recant?

That's fair. Although, now I'm unsure if you're implying whether Vatican 2 is not a heretical council by imparting a false understanding of religious liberty to those who embrace it.

It's not really relevant to the discussion here, since if Siri materially held the office, then he held the office.

I don't disagree. I'm just not really convinced anymore by the Siri thesis. Sure, he may have been legitimately elected in 1958, and there's some evidence to support that. But beyond that point, everything else is hearsay and reading into Siri's interior forum on whether or not he was the Pope. He never explicitly confirmed or denied that.
It still does not change the fact that he embraced the Council, said the New Mass, and enforced its new teachings against those traditionalist clergy.

Yes, Siri IS the answer to the mystery of iniquity in the Church, the Pope who suffers much. We also will never know until it is revealed what horrors he was threatened with if he spoke truthfully to rightfully  ascend to his Papacy. Millions killed by nuclear weapons and sent to Hell? Rome bombed, family murdered? Even if he should have risen above these things in faith and claimed his rightful position, we can't actually fathom what he faced. 
Siri can be accused as having a lack of courage and moral weakness, but that doesn't disqualify his valid election. As he never implemented His Papacy, any "heresies" attributed to him, which I doubt there are, are moot at this point. 

Did he embrace them?  Secondly, no, religious liberty as a term is not heretical.  1) it's never been defined with the note of de fide and 2) there's probably a different understanding of the phrase depending on who you ask.  I've seen a lot of people (conservative Novus Ordites) asser that it basically means religious toleration.

I think we just had a round of where I was defending you against the charge of heresy from RomanTheo.  When we asser that someone is a heretic, we'd better have 1) chapter and verse of which dogma they're directly denying and 2) proof that the person denied it.

To charge Siri with heresy, I'd need to see 1) citations from Siri himself and 2) an explanation for why it directly contradicts something that's de fide.  And then if you confronted him about it, would he recant?

It's not really relevant to the discussion here, since if Siri materially held the office, then he held the office.

So, how is he any different than any of the other Vatican II "popes"?  And if he isn't any different, then how is he "the answer" or "the key"?

That's fair. Although, now I'm unsure if you're implying whether Vatican 2 is not a heretical council by imparting a false understanding of religious liberty to those who embrace it.

Religious Liberty is not heretical, a grave error (which the Holy Spirit also would not allow a legitimate Ecuмenical Council to teach), but not strictly heresy.  Chief heresy of V2 was the false ecclesiology and and the denial of EENS ... which, sadly, most Traditional Catholics also hold.

He's different because as the legitimate pope his Magisterium would have been protected by the Holy Spirit.  God would not allow a legitimate pope to teach error.  But an imposter doesn't have the same protections.  This really isn't that difficult.

I need to add that these enemies deliberately waited for Siri to be elected and then to accept before moving in with their threats.  This was necessary, because they knew that the Holy Ghost would prevent a legitimate pope from teaching error to the Church.  Were a secret evil infiltrator to get elected, God would strike him dead if necessary to prevent him from teaching error to the Church from the Holy See.  Had they threatened Siri into refusing to accept, despite the threats, he would not have ever held the office.  It's only a resignation from office that threats invalidate.  But he does not hold the office until he accepts.  By pushing aside a man who was elected and accepted, they forced him out of office illegitimately under duress, which then impeded a subsequent election from being legitimate.  Consequently, they foisted an illegitimate pope on the world who was therefore not protected by the Holy Ghost and could proceed to wreck the Church.

So these enemies of the Church have more faith in the protection of the Holy Ghost over the Church and the papacy than R&R do.  This was a brilliant tactic, and they pulled it off flawlessly.