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Church History examples:
1.  Martin Luther (one of the most famous and public heretics in all of history), even after nailing his 100 thesis to the door of a church, was not excommunicated until a OVER A YEAR later, after multiple talks, hearings, etc.  Does this matter to some in our day?  No.  They can throw out any and all persons they deem necessary.  No matter that they are simple laymen, monks, or priests, with no jurisdiction, no canon law training, and not one iota of ecclesiastical authority.

2.  Those who correctly point out that Fr Feeney's excommunication was faulty (and therefore probably null), in that he requested a public hearing with rome officials (as is his right under canon law) but was ignored.  Yet these same people don't blink an eye when it comes to deciding that this or that priest, bishop or pope is no longer in office, ignoring all pretenses of a trial, or canon laws, or any kind of procedure whatsoever.

It's shocking in its lack of common sense.  It's a (sad) triumph of theory over reality, where "imaginary righteousness" takes precedent over the laws of justice.

Here's the thing that neither of you don't seem to be getting, and I apologize for the lack of clarity: I am operating on the principle elaborated by St. Robert Bellarmine in De Romano Pontifice, II, 30:

“… for men are not bound, or able to read hearts; but when they see that someone is a heretic by his external works, they judge him to be a heretic pure and simple, and condemn him as a heretic.”

A point which has been codified in the 1917 Code:

Canon 2200 § 2. Positing an external violation of the law, dolus [i.e. malice] in the external forum is presumed until the contrary is proven.

It is not a simple matter of the sin of heresy, as it is undeniable that heretics can become Catholics again and have a right to the Sacrament of penance. But a matter of the state they occupy which ipso-facto eliminates their exercise of office through excommunication:

Canon 2340 § 1. If anyone from an obdurate spirit stays for a year under the censure of excommunication, he is suspected of heresy.

§ 2. If a cleric stays for six months under the censure of suspension, he shall be gravely warned; and if, a month from the warning having passed, he has not withdrawn from contumacy, he shall be deprived of benefices and offices that he might have had in the Church.

The fact that these Antipopes have committed many heresies in the external forum, as Stubborn admitted that they are heretics, my presuming their loss of office follows on this fact because their obduracy has persisted for years. It is not a matter of waiting for a formal declaration, or canonical judgment. Once one is seen to be a heretic in the external forum, then it is acceptable to condemn him as such.

And given the Church teachings I have quoted previously, these men are firmly outside of the Church and cannot legitimately hold their offices.

1.  Martin Luther (one of the most famous and public heretics in all of history), even after nailing his 100 thesis to the door of a church, was not excommunicated until a OVER A YEAR later, after multiple talks, hearings, etc. 

Just because his departure from the Church wasn't formalized until a year after he initially defected from the faith does not mean he was still a member of the Church (Siscoe & Salza are completely wrong).  St. Robert Bellarmine cites Pope Celestine, who wrote of Nestorius (who was only formally condemned about 3 years after his defection), that he ceased to have any authority from the moment that he began to "preach" heresy ... and that all his acts had been null and void.

This distinction actually speaks nicely to sedeprivationism, where the man had lost all authority (formal) even if he wasn't formally excommunicated / removed from office until later (material).

It would, if it we were talking about a valid Pope falling into heresy and then losing office. The problem I see is that these Antipopes were never valid material to begin with, being manifest heretics prior to "election", and therefore have no office to speak of. Essentially the whole "impounded Pope-elect" idea I've seen some sedeprivs propose, specifically for Roncalli, Montini, Luciani, and Wojtyla, being valid bishops elected by true Cardinals; albeit, again, without office because of their manifest heresy. Ratzinger and Bergoglio, being that one is a valid priest but not bishop, and the latter is neither; cannot, from what I infer here, be proposed to even be Pope-elects.

So, perhaps the sedeprivationist thesis may have been possible for a time, say, up until JPII died. But after that point, you find yourself simply with a sede vacante.

Well, I don't believe they were even materially valid due to the office being materially held by Cardinal Siri (Pope Gregory XVII) until his death in 1989.

But there's a clear reality to the material-formal distinction.  Let's say the Siri thesis is not correct, these men would have had the Church's designation to the papacy ... which was never repudiated.  Had they converted some time after their election, they would then have formally assumed the office.  Heretics cannot formally assume or exercise office, but they can in fact be designated for office.

We have many heretical bishops even before Vatican II.  Let's take Cardinal Cushing, for instance.  Obvious manifest heretic.  Due to the fact that he remained designated for office by Pius XII, he did in fact to some extent remain in office.  He was able to still serve as a conduit for jurisdiction for the priests who were subject to him.  He could appoint priests to be pastors.  And so on.