You can work around that actually. This is no normal tadpole. So upon death it gets bound to your SPIRIT when you die as well. And reforms inside of your head upon resurrection.
Easy. Keeps immersion intact. A normal tadpole you can just crush. But THIS tadpole? It's some dark voodoo. Even the hag is afraid of it. And she's prepared for death. She shows her fear of the tadpole after you strike a deal to remove it and she flat out refuses. And if you kill her she's all "I was prepared to die".
The tadpole is scaring the hag more then DEATH. And the hag was prepared to die. But dangle an alien tadpole in front of her and she's going "F that, I'll just talk from my grave."
If you talk to the dead Hag with speak with Dead, she suggest she will comeback, aka be resurrected. She's not afraid to die because she knows she won't be dead long.
Hags always live in coven, so she's not actually alone. Going by her letters, her coven is in Baldur's Gate.
Yes, but that just means she's still afraid of the tadpole. And had it been a NORMAL tadpole it would have been fine. But THIS tadpole terrifies her.
So she's prepared for DEATH of all things but not "some alien tadpole". Just how afraid do you have to be of something that even death is preferable? With how quickly she wanted it away from her once she found out it's like she was worried about her own safety. Perhaps because it could affect her in or beyond death itself.