izze Looking great so far!
Your belly volume issue sounds interesting to me, as I wouldn't expect non-intersecting geometry to sizzle in the way you described.
If you still have the scene that exhibits the issue, would you either be willing to share it, or take some screen grabs with all of the debug drawing visible?
(Usually the kind of sizzling behaviour you described happens when you have collisions that fight against attachments, and or due to having not enough degrees of freedom - meaning not having enough tetrahedra in your tet mesh to resolve the demands of the attachments, collisions, gravity and any other forces that might act on it.)
Cheers!
-jj