One potential issue is that the MDL files referenced in your USD files are not next to your USDs files, actually they are not in the archive you sent with me at all.
Example: uniform asset info:mdl:sourceAsset = @OmniSurface/OmniSurfaceBlendBase.mdl@ and uniform asset info:mdl:sourceAsset = @nvidia/core_definitions.mdl@.
So either you are missing files, or they are in a path that USD in UE does not manage to resolve and thus UE fails to find the files.