But then I wasn't terribly impressed with how Gratiana was handled anyway; I would really have hoped for more conversation options to confront her about her past.
This.
I had so many questions for her.
In the end, I broke her soul jar in front of her. She wants her suffering to end? Releasing her soul just do that. For some reason she wants to stay "alive" after 1k years of being an undead. I find this -at least- very suspicious.
If she's honest with you and tell you the truth she have nothing to fear when you break the soul jar.