Whew that lamia amirite? Tough cookie but I think if you level up your struggle enough you should be able to escape. I was never able to but I loaded into a new save and went straight to her without leveling up first so that was probably a problem. I think a way to get around the eternal lamia belly prison is for Amelia to be "digested" and then reformed at the starting point that way you can go to other preds and level up first.