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.

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.

I have read about the "Siri thesis", and I think it's a foregone conclusion that he was elected in the conclave, but what was supposed to be so terrible (in the eyes of his enemies) about Siri being Pope, that would prompt such dire threats and blackmail?  What was he going to do that Pius XII had not already done?  To whom did he pose such a threat?  If nuclear war were on the table, the Soviets were the only ones who would have had this capability, and would have had a reason to be threatened.  Or, for some reason, did the US not want him to be elected (Fɾҽҽmαsσɳɾყ?), and they would have been the ones to use the nukes?  Against Italy, an ostensible ally?

Or was it more a case of "because he's not Roncalli"?  Because the enemies of the Church wanted Roncalli, and that Siri's election thwarted their plans? 

If Siri's resignation was not valid, then he would have remained pope, since he was canonically elected, correct? Sort of like Benevacantism . . . Sirivacantism.

Yeah, maybe.  But nothing else would be "helpful" in your case either.  You have these emotional reactions (and therefore won't consider rationally) anything that doesn't fit in with your straight sedevacantist viewpoint, from your animosity right out of the gate against Archbishop Vigano, your railing against sedeprivationism, and now this.  You won't think in terms of distinctions, and that's the key to your thinking on all these issues.  You only think in binaries.  Either someone is the pope or he isn't, vs. he can be a pope in one respect but not another.  +Vigano bad because he hasn't declard the Holy See vacant and he used to belong to the Conciliar hierarchy ... and similarly with +Siri.

Not everyone who belongs to the Conciliar Church is a formal heretic, and it is possible to be elected to or designated to office before being able to formally excercise its authority.  If a layman were elected and accepted, he'd be legitimate holder of the office at that moment, but he could not actually exercise the teaching authority of the office without being ordained and then consecrated.  That distinction can actually be found in St. Robert Bellarmine.

But anything that doesn't fit in with your simplistic vision of Novus Ordo bad and See vacant, you reject and rail against without making any rational arguments, and then when others present them you ignore them or filter them out.

So it's gotten to the point that I don't really need to be "helpful", since you don't want to be "helped" ... in other words, you refuse to think outside of your own personal conclusions and there's no real point discussing these matters with you.