So if UPA is true then all those V2 popes were valid popes, utterly destroying the sedevacantist position. 

No.  A validly elected pope can lose his spiritual office (or it can be impaired) for a number of reasons.  UPA doesn't destroy sedevacantism at all.  It is only concerned with the election of a pope.  His orthodoxy is another matter altogether.

No.  A validly elected pope can lose his spiritual office (or it can be impaired) for a number of reasons.  UPA doesn't destroy sedevacantism at all.  It is only concerned with the election of a pope.  His orthodoxy is another matter altogether.

For a Pope to lose his office, you'd need at least a small minority of Bishops to stop commemorating him. That's what Cardinal Billot says in the case of Pope Alexander VI. It's why, very likely, a Pope can only lose his office after warnings from the Cardinals and Bishops. At that time, after the warnings, he is no longer universally accepted, but many Bishops don't acknowledge him. Then, perhaps he could fall from the pontificate. 
It is certain that every Pope up until Pope Benedict XVI was validly elected and remained Pope until their death/resignation by this criterion. Whether Pope Francis will remain Pope or not until the end is something only God knows, I think, as it depends on the future.

Devil's Advocating, this has happened.  There are a couple bishops in the SSPV, at least one in the CMRI, Donald Sanborn.

Why don't they count?

They are not really Bishops in the full sense because they have no authority from the Church. They are just laymen who managed to convince aging Bishops with traditional tendencies to give them orders. If I went to Bishop McKenna or even a retired Novus Ordo Bishop with a traditional Consecration and convinced him to consecrate me I would have just as much authority as Bishop Sanborn. That is none. I don't know how exactly to word it, but they have orders but no jurisdiction as they say. 

So every pope since Vatican 2 was validly elected and then was deprived thereafter? Well, it might work with sedeprivationism, but it in no way aids that position or any sede position for that matter so it's hard to see how sedes could have any motive in pushing it.

Sedeprivationism is the only theory which lines up with all/most of the facts.  The election-then-deprivation-of-spiritual-office lines up with the fact that both St Pius X and Pius XII changed the election laws to allow exactly this scenario.
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Meanwhile, sedevacantism has a plethora of reasons why the V2 popes aren't popes, with no consensus on the main reason, with no explanation of the above election law changes, and mostly using indirect logic (i.e. working backwards.  Example:  Pope was elected, pope spoke heresy.  That's not possible, therefore pope wasn't elected in the first place).  R&R also does not explain the election law changes, because they (generally speaking) don't acknowledge that the V2 popes are 100% heretics.

That's true for the actual, physical, material, governing office.  Not true for his spiritual office.

There's no difference between an actual, physical, material, governing office and a spiritual office.   It is all one and the same.  The distinction was invented by the heretic des Laureirs.  No real Catholic, much less one who called himself a Traditional Catholic, would give this novelty a second thought. 

There's no difference between an actual, physical, material, governing office and a spiritual office.   It is all one and the same.  The distinction was invented by the heretic des Laureirs.  No real Catholic, much less one who called himself a Traditional Catholic, would give this novelty a second thought.

That’s a strong accusation. Can you provide proof that he was a heretic?

Sedeprivationism is the only theory which lines up with all/most of the facts.  The election-then-deprivation-of-spiritual-office lines up with the fact that both St Pius X and Pius XII changed the election laws to allow exactly this scenario.
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Meanwhile, sedevacantism has a plethora of reasons why the V2 popes aren't popes, with no consensus on the main reason, with no explanation of the above election law changes, and mostly using indirect logic (i.e. working backwards.  Example:  Pope was elected, pope spoke heresy.  That's not possible, therefore pope wasn't elected in the first place).  R&R also does not explain the election law changes, because they (generally speaking) don't acknowledge that the V2 popes are 100% heretics.

All fair points, but it doesn't address the actual point of contention - UPA in no way aids the sedevacantist position and therefore it does not make sense that sedes would be the ones pushing it. 

As I explained earlier, and with Clemens Maria's post on this thread being an example, he used the UA as a "gotcha" test against (presumably) R&R folk.  It has no bearing on the sede theory, but it's just used to attack the "other side".

You are a stupid idiot.  I don't even know if Universal Peaceful Acceptance as interpreted by the neo-SSPX is even Catholic teaching never mind believing that it is applicable to the current situation.  At no time did I ever argue for UPA (or UA, whatever).  But you are too stupid to understand that.