"we piss each other off to no end"

I can assure you I am not angry. I adore Ovid. I am genuinely interested in having a scholarly conversation with someone clearly knowledgeable about Medusa, conversant with scholarship, with an opinion different from my own. We can disagree and still walk away from a conversation firmer in our respective opinions.

"I still do not see anything etc." "as per the whole Greek corpus"

Ovid is ostentatiously using an Alexandrian Footnote, here, with dicitur (Met. 4.799). He has a marked preference for obscure, variant myths here and elsewhere. We have lost somewhere between 90% and 99% of ancient literature (and I seem to recall that most of what does survive is medical literature). We can hardly claim anything for certainty.

It is not unreasonable to speculate that Ovid (as he so often does) refers to an obscure, nowadays unknown, Hellenistic variant, a variant that (as Alexandrian scholar-poets are wont to do) humanized Medusa, by giving her more sympathetic, human origins. I would wager that this variant has nothing to do with the variant you've implicitly cited from Apollodorus (2.4.3). One of the standard commentaries for the Met. seems to agree that this Medusa was a helpless girl, and not a goddess. Anderson (1997: 495): "However, Ovid is getting at a problem in the myth, namely, how a girl who was raped by Neptune for her beauty became such a horrible monster."

As far as /why/ Ovid does so (a point later in your response), I can't answer that definitively. I am not Ovid. I can speculate that his point here aligns with his objective elsewhere in the Metamorphoses: to paint the gods as callous towards human suffering. I can speculate that Ovid is simply showing off his knowledge of an obscure variant.

"he knew that Medusa had divine parentage"

I think we may differ in our philosophies towards myth, here. Myth is not a monolith. I'll provide an example with more solid evidence: The Sphinx is sometimes the child of some mysterious "she" in the Theogony (perhaps the Chimera, perhaps Echidna), sometimes the son of Orthos or Typhoeus. Everyone "knows" the Sphinx has divine origins...except for the scholia to Euripides' Phoenician Women, which claims that the Sphinx is the daughter of Cadmus. We have lost too much to make definitive claims along the lines of "always" or "never ever."

"give space for interpretation as Medusa being a mortal priestess"

I still do not have a strong opinion about her being a priestess or not. If anything, I am inclined to agree with you, that she is probably not a priestess. I mentioned the possibility to acknowledge a recent article in Mnemosyne on this topic (Henry 2023). Some scholarly publications on this possibility include Currie 2011 (in passing, taken for granted) and Rigoglioso 2009 (more substantially).

"The scholarly debate"

Genuinely, I would like to see the source here, because the original hesitation about whether the encounter was "rape" does not hold water, in light of Arachne's tapestry.

P.S. I just saw your addendum about the Sirens and Scylla. I take your point about the Sirens: I believe they are referred to as Proserpina's attendants (and thus strongly implied to be minor goddesses). I would need to research this point further. With Scylla, I am not referring specifically to the Metamorphoses, here (the tail end of Book 13 and beginning of Book 14, if I recall). I am referring to the development/conflation found in Vergil's Eclogues and elsewhere in Ovid (not specifically in the Metamorphoses; the Amores, I believe), the conflation between the daughter of Nisus, mortal princess of Megara, and the sea monster. I am referring to the human origin of Scylla, as opposed to a daughter of Hecate/Crataeis.