Thanks for your patient tutorial about this compatibility problem! Just curious about why such problem exists between these good partners(U3D and blender)? Hope some add-on could be made to replace such tedious procedural in the future.
I'm glad it helped! I do hope the relationship improves in the future, but in this case it's on Unity to fix the importer. If you're using an external texturing tool like Substance then this workaround isn't necessary since it imports 32 bit perfectly and exports for 8 bit just fine.