"Without a champion, the gods are powerless to act."-The Prophet.
now, its not all that much related to the question you ask about the Prophet, but i have to say, that is some statement there. it is stated with enough finality to it that it leaves no room for multiple interpretations, don't you think? in other words, even if they wanted to do something to stop Umaril meddling, they still need a mortal to do stuff, anything, or they are powerless to act. very limited power for the gods is they are powerless to act, like, just barge into the mortal realm and kick Umaril's you know what. most curious.
now Prophet is a... a speculations list is in order, i am not knowledgeable enough of Lore to draw definite conclusion. i just think he is a tool. in more meanings than one, maybe. anyways, the speculations list:
a)madman(formerly a clergy of sorts, or perhaps just learned enough about religions or something) gods use to get CoC's attention.
b)normal man(clergy of sorts, or perhaps just learned enough about religions or something) that gods use to get CoC's attention.
c)avatar of a god... a possibility, yes, given Bethesda made him vague enough and so open to interpretation, what with all the secret knowledge you mentioned and the blessing of Talos and all that.
considering my lack in knowledge of Lore in regards to the Prophet and that Bethesda provides no canon to some things in Lore, i think he is just that, another non-cannon-ified NPC that's used to entice CoC to undertake the quest and everything else is something for you or me to jump start our creative mind and come up with out preferred interpretation of who or what he is. or to write our own fan fiction about him and or who or what he is
.
CoC a reincarnation of Pelinal Whitestrake? i am thinking no. but then again, i am not enough familiar with all the Lore, not enough to say if CoC is a reincarnation of Pelinal Whitestrake or not. Pelinal appears to you in a vision. neither The Prophet not Pelinal mention reincarnation. do any prophecies or other sources even imply reincarnation of Pelinaland? i don't know if they do, so that's why i am asking. Pelinal, amongst other things, says "Your need must be great for the gods
to allow us to speak". to me, sounds like an implication of his identity being intact and that CoC is not him. or the gods are just messing with CoC's head and whatever it is that appears to CoC just poses as Pelinal.
if CoC is Pelinal 's reincarnation... communicating to self? normally not. but then again, if CoC is on his way to insanity, then who knows.