Each object on the surface should have its own collision. So that when it rotates in place the character still won’t be able to walk into it.
and yes, they should be separate from the base object too, mostly because you really want the engine to occlude them automatically.
As far as objects with physics. You can.
You just need to enable physics only when the character is close and disable them as the object’s rotation tilts past its tipping point.
you can actually calculate this with basic physics math. If you know the object size (bounds), its local center of gravity, you can determine at which angle of incline the simulation would tip the object over. I would say to turn it off about 10deg. Before the tipping point is mathematically reached to be on the safe side.
though, you should be aware that the rotation of the base may impart extra force to the math.
As far as the collision goes.
Let me point you to some intel optimization articles on how to efficiently split meshes for occlusion.
Doing this should actually “convert” the object to where each piece can be a convex hull.
I would also LOD manually at least 2 levels to add/reduce detail. Using the highest lod for collision purposes.
here is the article. Scroll down to the occlusion part. They have image graphs on different ways to slice stuff.