If your character has facial bones and you want a modular design, it’s better to export head skeleton separated from the body’s. Perhaps that is the reason for the “merge errors”. Take a look at Metahuman Sample project and see how it is setup. So, all body parts and accessories will share body skeleton: shoes, hats, helmets, backpacks, clothes, etc. Hair meshes and assets will use Head skeleton, including the alembic groom assets.