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And I might be missing something here, but I thought all Papal writings are contained on the Vatican Website?  For some reason my searches for this docuмent on the Vatican website are coming up with nil.

Vatican website papal docuмents don't go back to 1559. Try the PDF at this link: https://www.todayscatholicworld.com/cuм-ex-apostolatus-officio.pdf

and in Latin at: https://la.wikisource.org/wiki/cuм_ex_apostolatus_officio

After reading Fr Cekada's great response to the R/R position of the mid 90s, he brought up Paul IV.  How does sedeprivationism get around this from Cuм Ex Apostolatus Officio?

This has been addressed many times.  cuм ex is a legislative / disciplinary decree, and not primarily doctrinal (though there are some implied doctrinal principles), and the docuмents issued by St. Pius X and Pius XII seemed to indicate otherwise.  What Paul IV was doing here was in fact by his decree pre-emptively removing them from office, thereby also removing the formal link to offices held by manifest heretics ... but it does not run counter to sede-privationism, but in fact reinforces it.  Sedeprivationism is also implicit in the writings of St. Robert Bellarmine, where he distinguishes in the case of Nestorius someone who lost authority since he began to preach heresy (aka manifest his heresy pertinaciously) losing authority while being in a state of excommunicandus ... relying on the teaching of Pope St. Celestine.  So Nestorius lost all authority, but was not officially removed (excommunicatus) but rather in a state of pending excommunication excommunicandus.

Straight SVism is untenable due to the argumentum ad absurdum of allowing Father Cekada's "Aunt Helen" to wake up one morning during the reign of Pope Pius XII (or a future Pope Pius XIII) and decided that the See is vacant because she had discovered heresy in his teaching.

This is a problem that has never been adequately addressed by the straight SVs, and something that Bishop Sanborn also came to understand.  There has to be some principle to prevent this type of "chaos" in the Church (as John of St. Thomas characterized it), and that principle is not clearly articulated by SVs.  See, one could argue from Universal Peaceful Acceptance, but then UAP seems to fly in the fact of SVism, since Roncalli and Montini (at least in the beginning) were certainly universally accepted.

The argument to this is that Aunt Helen could just be plain wrong and context matters.  If I wanted to go through let's say...Pope St. Linus's teaching and take something out of context and find "heresy" then the right way to handle that is to say they are wrong on the matter and to recant and in charity show them or tell them why they are wrong.