OpenMW does not handle RootCollisionNode correctly
I just found out that for the OpenMW engine it matters to what is the RootCollisionNode linked in the nif file hierarchy, while for vanilla engine it does not, in order to have reasonable collision.
Many mesh replacers don't have the RootCollisionNode linked to the root node in the nif file hierarchy, in OpenMW this causes them to not only use their collision box, but also to use the regular mesh as collision. (see Collision_OpenMW_badcollision_mesh.png)
When you attach the RootCollisionNode to the root node directly, the issue is gone and the mesh uses only the collision box in the OpenMW engine. (see Collision_OpenMW_goodcollision_mesh.png)
In vanilla it does not matter to what is the RootCollisionNode linked, it always uses the collision shape only. (see Collision_vanilla_badcollision_mesh.png)
You can test this with the attached meshes (drop them into meshes/f) and observe by using the tcg or tcb commands.
The OpenMW engine should not use the regular mesh as collision when the RootCollisionNode is found in the nif hierarchy.
There were quite a few revisions to that file, and the fact we only look for direct children of the root node is most certainly an intentional change intended to fix some other model. I suggest to look into git history and do a lot of testing with other models first.