The history of the gods may be unknowable, the history of the origin of the modern cult seems much open to investigation, as described in the bit of BoneyM's post you didn't quote:
Even if there was Belthani worship of a god that can be idetified with Taal, that may not matter. If the modern Cult is descended from the Taleutens as described in Tome of Salvation, after the role of priest split off from the role of king (page 12) then there may be no continuity between Belthani worship of that entity and the current Imperial cult. The way the text is written in such an over the top way strongly implies that the 'Tilean' theory that the institution/role of priest was something that the proto-Imperial tribes picked up from the southern tribes is true.
Generally, this section of Tome of Salvation, is, I think, some of the best Warhammer material and is well worth a read if you have access to it. An interesting bit I just read is this:
You can see why Cython thinks that some of the human gods are the same as the elven ones with those early names!