A great, ambitious game. Very moody and mysterious and I enjoyed the adventure even though it was a little short. It was a pretty solid experience for me since I didn't encounter any major bugs.
I'm not sure how familiar you are with physics engines but you could probably achieve more stable collisions by using bunches of simple primitives as collision shapes (box, cylinder, sphere or whatever else Bullet supports) instead of using the render meshes as the collisions. I mean, if you're not already doing so. If not, you should definitely look into it. I'm not familiar with Bullet physics engine specifically so I don't know the details of how to set it up though but the same principles usually apply in all the major physics engines.