Siri was not a "liberal".  He called V2 the "greatest disaster in Church history".  He was friends with Archbishop Lefebvre and part of the Coetus Internationalis Patrum, and was always characterized by biographers as a "staunch conservative".  So he was hardly a "liberal".  Whatever errors he may have made in judgment were done in his capacity as a private person and not in exercising his office (which he did not do).  No one has yet demonstrated any manifest heresy that Siri adhered to that would have rendered him a non-Catholic and therefore to have vacated the office ... because no such example exists.  He could even have been mistaken in thinking that he resigned, not considering the fact that the resignation was illegal due to it having been under grave duress.  Mere acquiescence to the NO "hierarchy" does not ipso facto make someone a heretic.

Just imagine for a second that you're Siri.  You are elected pope, accept, take the name Gregory XVII.  Then a group of Cardinals come by hurling threats, perhaps it's to kill off all the bishops and priests behind the Iron curtain, perhaps it's to kill your entire extended family, perhaps to nuke the Vatican, and you give in, "Fine, I'll resign.  Pick someone else."  From your entirely subjective perspective, you resigned.  But viewed objectively, by a third party, the resignation was not free but made under grave duress, therefore making it invalid/illegitimate.

Pius XII, Mystici Corporis

For not every sin, however grave it may be, is such as of its own nature to sever a man from the Body of the Church, as does schism or heresy or apostasy. 

Shoot, this went off the rails

This is not factually accurate.

The Friday discipline continues ag least on paper in the Conciliar Church. However, in most jurisdictions the obligation only binds under pain of sin during Lent.

An example where Friday abstinence still obliges year-round under pain of sin would be England & Wales. Yet, if one crosses into Scotland the obligation under pain of sin is only during Lent.

Plenty exists about which we can criticise the Coniliar Church. We do not need to exaggerate which only makes us look foolish.

Thank you.

I will need to learn more about Cardinal Siri.

100. Post electionem canonice factam, ab ultimo Cardinali Diacono accitis in aulam Conclavis S. Collegii Secretario, Praefecto Apostolicarum Caeremoniarum atque duobus Caeremoniarum Magistris, consensus electi per Cardinalem Decanum nomine totius S. Collegii his verbis: Acceptasne electionem de te canonice factam in Summum Pontificem? requiratur.

101. Hoc consensu praestito intra terminum, quatenus opus sit, prudenti arbitrio Cardinalium per maiorem votorum numerum determinandum, illico electus est verus Papa, atque actu plenam absolutamque iurisdictionem supra totum orbem acquirit et exercere potest.

Siri theory

Just because your friends, Pope Paul VI and Pope John XXIII were actual popes that were heretics is too much for you to bear, there is no need for you invent this so called theory that a different NO Cardinal named Siri was the real pope. Pitiful actually.