perhaps her tadpole’s original host was killed in the fight with the devils or githyanki and desperate for a new body somehow crawled into her before she realised what was going on.
This sounds good enough to me.
My own theory on the Karlach stuff is that she boarded the nautiloid while it was in the hells in the prologue, and was possibly part of Zhalk’s boarding party
This was my original theory aswell ...
But it have some problems.
First, as you mentioned it seems oddly fast.
Second, Karlach herself say something like "if you would seen as much Avernus as i did, you would rather risk having a tentacle" or something among those words ... implying her tadpolisation was willing, or at least not as accidental.
Third, in game intro our PC get tadpoled before we even reach Yartar ... meaning quite some time before we went into Avernus.
And finaly, there is allways chance that she just lie.
Dunno, feels like a plothole so far.
I really hope we get whole story.
