It is not, as far as I can tell
I'd imagine so, but that's the kind of thing that is up to the DM. Proteans are not available to be PCs, so rule 0 (whatever makes for the best story) applies, both when used as an antagonist in an actual game, and in OotS. We could posit, for example, that given MitD's vague (repressed? Lost to time?) memories of the astral plane, he might have spent some time there, and met creatures that move through it on their way to other places. That'd give him a fairly solid set of creatures to borrow Ex powers from.
Grey Wolf