[Note: I think this might be the point you were making above, but I have failed somehow to prevent overlap, despite using “Simple as Complex” and trying to move the vertices of the meshes so they don’t overlap. I have also triewd using the actual mesh as the hitbox without success.]
The issue is not with the meshes but their proximity to one another.
It seems Geometry Collections cannot be near one another. This is true even if the vertices have been separated so that they don’t overlap i.e. even when the models are separated and hitboxes do not share space 
.
If you have multiple meshes and drag them out of the content draw together, so they enter the viewport world correctly arranged, they then do not work as destructible geometry collections 
.
A single mesh, extracted alone from the content draw, functions as expected.
See below for a demonstration…
Single mesh compared to multiple meshes:
How each were extracted from the content draw:
I cannot find a way to view GeoCollection collsions.
I’m not expecting you, MostHost_LA, to solve this for me I’m just keeping the thread alive in case I find a solution and someone else runs into the same problem.