Thank you for writing this up. Minthara does not clue you in on anything and thus comes across as a one-dimensional villain nobody in their right mind would betray everyone they have met so far for.
I don't see one-dimensional as a problem. That can mean focused and efficient. Her focus is obviously gaining the favor of the absolute. I think the real problem is that she does not tell the player why working for the absolute would be a good deal in the players situation. "Don't worry, no true soul has ever turned into a mindflayer" would be a start. Larian obviously did not want to spoil too much about what the absolute is and what its goals are. That creates problems. "Hey there stranger, work for the NTFQ on some high risk job" "Why and what is the NTFQ" "Some intelligence agency somewhere with some purpose. I can tell you nothing about the details of the mission, our country or the salary. Are you in?"
Make the dream or the encounter with Edowin point the player toward the absolute option
Explain at least a bit about what the absolute is
Explain why the player does not need to worry about turning into a mindflayer by abandoning attempts to ged rid of the tadpole
Explain what the absolute has to offer and why it is likely to win the fight with the other religions and establish dominance in Faerun
