Damage from regular attacks won't persist when Marco uses regeneration. Only Haki damage does. King's flames are working on Marco and damaged him, so it doesn't always apply. It's just that Big Mom's flames aren't strong enough for him.
Im thinking some of these seraphim if not all of them, will become their own person later in the story. I dont see this authority the WG has over them, stay intact as the seraphim continue to grow. As of right now, S-Hawk & S-Snake are the likely candidates to break control.