My read on this:

The parent sacrifice is a trap. She's free of the pain but pain is Lovitar's domain, not Shar's. Shar is the god of loss.

So if she goes through with the sacrifice she needs to learn to embrace loss - the same contemptuous thing she said to Viconia. And when she loses Tav she experiences her second great loss.

She's the new Kethric Thorn - her life has dismantled bit by bit and the moon maiden's cold light offers no comfort. Better to embrace loss and hope Shar will take those memories from her.

Which is clever - I think it's an example of good writing. Someone really thought about Shar's divine domain.

But it's also another place where the railroaded mindflayer requirement is undoing your actions. The devs need to remove the forced transformation from the game.