It doesn't make sense.
By the book lore Gortash knew that the prism disrupt ilithid powers. Yet he sends an ilithid which is only kept in check by elder brain mind control to get it.
Neither does the timing of the tadpoling. When did the emperor even abduct the party? Most of them were in Baldurs gate and you don't just fly into Baldurs Gate with a Nautiloid. Especially Gale could have blown up the ship on his own if it tried to openly abduct him.
And if they were abducted before, why weren't they tadpoled before the mission instead of after the raid when their support was needed most?
That Mephi simply allowed Gortash to take the crown also makes no sense. What would he possibly gain from that?
Agreed - why would Gortash send the Emperor to retrieve that Prism? My only theory is, that he knows, it could be bad for the plan, but not how.
This book lore is the ground for that theory:
The Astral Prism Heist - but he at least knows, that the prism disrupts illithid powers, so sending an illithid force was a big gamble.
Mephistopheles would have crushed Gortash and Durge for entering Cania and trying to steal the Crown. No way, they could have left with it without him wishing for them to leave. It made me think, if there was more planned with Mephistopheles and then scratched last minute.
I've read up on hell lore a bit, since I play DnD for 20 years, but never had a plot involving the hells and thus wasn't that versed. But according to the lore, Mephisto always is up for magical experminets and Cania is riddled with magical traps. So I can totally see him wanting to see, what can be achieved with the Crown and intervening if he thinks, he can something out of it.
