Dog Moon -
That's acceptable. No changes after you actually start casting spells, though.
Voidrazor -
I concur. FoP, however, is allowed unchanged by precedent, and thus a nonissue insofar as consideration.
Regarding the definition of a fiend, it is certain that tanari'i, baatezu, and yugoloths are fiends, but it is less clear if evil outsiders are all technically fiends. Nevertheless, it is a moot point if Outsider (evil) is all that is required.
The main concern remaining is that I'm still not entirely sure as to the viability of having something so unique that it breaks with its base concept in several ways simultaneously - as a fiend of possession, as a phrenic creature, as an individual not living in a cyst, and as a divine caster. Let me clarify that this is not a matter of balance; it is a question of conceptual coherence. (If that isn't clear, consider that, say, a half-dragon, half-centaur, phrenic dwarf is also three degrees of variance - and not really much like what one thinks of when one says "dwarf."

) I am uncertain as to where exactly the line is between "complicated concept" and "unsightly assemblage of templates," but three degrees of variance seems to be close. Is it possible to reduce it to two?