Irrelevant.  We've discussed this 50 times.  But, as per usual, you just keep spamming the same tired old nonsense in over and over again, wasting everyone's time.

Not sure why you're still backing SSPX since you consider Archbishop Lefebvre to have been a heretic.

Siri may have thought he resigned while, under law, it didn't take effect due to duress.  The "threat" which made him resign did not necessarily have to be personal for it invalidate the resignation.  I've heard that he was told that the "communists" wanted John XXIII to be pope or else they were going to start a persecution of Catholics and WW3.  He was told that he was viewed as an extremist and that in order to avert global catastrophes, there needed to be a "moderate" pope. 
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If something like the above were true, then one could argue that Siri stepped down "of his own accord" and not under duress.  Legally they would be wrong, as duress encompasses both personal and non-personal reasons.  But I can see where Siri (and others) could make the argument that the resignation was done freely.  If Siri believed that he resigned for the "good of the Church" then he obviously would consider John XXIII to be valid, hence he would treat him as a real pope.

So as to not exasperate you, could you please provide a link to a page which you consider contains the best response/rebuttal to this letter of Cardinal Siri?

As it stands, I do not understand how, in the face of this letter, one can maintain Siri was nevertheless pope (unless one wants to argue he was oblivious of the fact, which is impossible).

Pax summarized the response to the "argument" implied in this article.  Basically the point of the lengthy article (which could have merely been summarized) was to say that Siri himself acknowledged the V2 Popes as legit.  To which, see Pax's post above.

If this is more or less the main argument of the Pope Siri advocates (an issue I have never studied because of its absurdity on the surface), then I begin to understand why they are so opposed to the idea (shared by the near-unanimous consent of the approved theologians) that the universal and peaceful consent of bishops infallibly ratifies and validates the pontificate of the elected (healing in the root any vice or defect of said election).

Well, the Siri situation is quite complex, so you can't apply the "universal and peaceful consent" doctrine as if it were a normal papal election.  Here's how it probably played out (based on the evidence we have of the white smoke and the rumors of a chosen papal name of Gregory).  If I knew the exact papal procedures/process you could probably re-enact the "crime" of threats/duress, but I don't know this much detail.  I'm sure it's in a book somewhere, as the papal process isn't a secret but only the voting is.  Anyway, here's how it could've gone done in real life.
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1.  Siri get the required votes and is elected.  This election happens in the main, large room of the Sistine Chapel, let's not forget.
2.  The "smoke" people are alerted and they start the smoke in the chimney.
     a.  The "smoke" people aren't in the Sistine Chapel, since that area is for Cardinals only.
     b.  They are in another part of the building and are notified by whoever cleric is the "master of ceremonies".
3.  Siri choses the name of Gregory and is ushered off to a side room, so he can vest and get prepared to go outside to the balcony to greet the people.
4.  As soon as Siri is out of the Sistine Chapel, away from the other Cardinals, he is threatened (or the Church is threatened) and he resigns under duress.
5.  Siri returns to the Sistine Chapel and announces his resignation to all the Cardinals.
6.  Immediately, the "smokers" are told to stop the white smoke.
7.  After the chaos and surprise die down between the Cardinals, they start a new election.  None of them would know about the threats/duress that Siri was under, since they weren't in the side room with him. 
     a.  Siri and all Cardinals are under vow of secrecy, so he wouldn't tell anyone, though later in life he made references to mistakes he made that he has begged God for forgiveness.
8.  When John XXIII is elected, all the Cardinals would think that his election is valid, not knowing that Siri's resignation was legally null and void.
9.  All of this could've happened in a manner of minutes, which would coincide with the newspaper accounts of the white smoke being only visible for a short time.
10.  It would've happened quickly because those Modernists/communists who were inside new-rome at the time, would've had this plan in place, as an "emergency measure" if an orthodox cardinal was elected, since he wouldn't go along with V2, which was the ultimate goal, and required getting someone like John XXIII elected.

But the theory of the “oblivious pope” (ie., Siri was pope, but didn’t know it) is certainly the wildest sede invention thus far, and makes the legitimacy of Pope Michael seem more reasonable in comparison.

Siri wouldn't be oblivious, nor would be (properly speaking) be the pope.  The Cardinals (all except for the evil ones who were in on the threats) would be (or could be) oblivious.  Siri would be the pope-elect only, since he never went through the coronation/mass process.  He would be legally/materially the pope, but maybe not have full spiritual authority (I'm guessing).  John XXIII would be an anti-pope (just like the previous John XXIII in prior church history...a coincidence that is too eerie to be a coincidence.)  John XXIII wouldn't have the spiritual authority, only the temporal/material office (even if by force and trickery).  In essence, the spiritual seat would be vacant.

Can you explain what's absurd about it?  What are your explanations for the following facts?
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1.  White smoke for a few minutes, indicating a papal election, then black smoke.
    a.  This has never happened before, since smoke was used for papal elections.
    b.  The Vatican has never given an adequate answer why/how this "mistake" happened.
2.  Unbiased, 3rd-party, FBI testimony says there was an election of Siri.
    a.  It is said that Siri took the name of "Gregory".
    b.  FBI wiretapping of "communist leaning" Cardinals proved that Siri was elected, since those Cardinals were illegally and immorally communicating with the outside world before,     during and after the conclave.
3.  Multiple interviews with Siri before he died corroborated the above story, if indirectly, since Siri cannot break his vow of secrecy directly.
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To date, there's been no other "theory" on how/why the white smoke happened.  In absence of another explanation, the above is the only theory which fits all the facts together.

Siri Thesis:  Absurd to the nth degree.

No wonder the human race will seek to annihilate itself, it is dumber than mud.

MO is that absurd to the nth degree == the idea that the anti-pope frank is actually true Il Papa...

Siri Thesis:  Absurd to the nth degree.

No wonder the human race will seek to annihilate itself, it is dumber than mud.

Would you mind phrasing this brilliant argument of yours in the form of a syllogism?