I think it has been stated that the player will visit Avernus again at some later point in the story.
Aye, stated or otherwise, it certainly seems that way. There is also the refugees-from-Elturel who are your friends from a good Act I, and hunted by Minthara/Absolute. Why? Why would they care? Note that one of the Tiefs has that coin, the one chased by an assassin.
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The Sharrans do not seem to be in a pact with the Absolute.
No, I doubt that, as the Sharrans would be one of the factions-to-oppose-or-join against the big bad. Like the Selunites, you can tell that they have been there before you, but they're already gone by the time you get there. Also, I doubt they'd be in league with Mind Flayers.
