I have a theory which resolves the conflicting Arkay origins of being an et'Ada and ascension-via-Mara.
The first Arkay is the et'Ada one that showed up with the original spirits. This Arkay, after giving himself up to help create the Mundus, was rather tired of being unable to manifest very much, and pulled a Mara by temporarily shoving his duties off to the other deities with similar portfolio (Mara probably used Dibella or Sanguine as a do-this-for-me. Arkay would likely use Meridia or Akatosh), then plagiarized the Black Book software off of Hermaeus Mora, wrote himself down as a book with the secrets of life and death, and had Mara give it to the guy who becomes the next Arkay somehow. Possibly by Ama Nin manifesting long enough to dump it wherever it was later found.
The next Arkay origin shows up here, with the guy-reads-book, Mara grants understanding, guy apotheosis mantling-and-merging scene, then suddenly the next Arkay.
The hypothetical next Arkay occurs if this fails and Arkay is not any more capable of manifesting. He continues the trend of stealing from the neighbors and nicks a spot in Florentius Baenius' psyche, probably right next to whatever aspect represents the little bit of old Sheogorath that lives in the brain of every mortal and possibly some deities. Arkay takes up residence in Florentius Baenius' mind, either masquerading as a tulpa or tulpa-like hallucination which supposedly just looks like Arkay but really is, or being straightforward and explaining his presence in the poor guy's head, or the new Sheogorath Hero of Kvatch being nice to Arkay and relaying messages in the guise of Arkay. Disregard if Florentius Baenius is later shown to be being fed messages by Sheogorath without any actual request of Arkay or is simply crazy and nobody's telling him anything, and he's just got lucky once or twice with the random dialogue and easter-egg-happy developers. Or Mai'q is a daedric prince, or he ate an elder scroll, or whatever Sheogorath-designed scheme of explanation and wild mass guessing you come up with.
Thoughts?