A few things about the proposal, assuming I understood correctly (correct me if not):

1. You are basically disallowing a region-name to be used for an entity and a collection. Right? That is not a valid change. Hibernate has (inadvertently) supported that for years. You can't just remove that capability, especially if y'all wanted this fix in 5.x bug-fix releases.
2. As-is your "disallow incompatible regions of same name" manifests as a NPE in the various CacheImpl#determine {TYPE}AccessStrategy methods; you have CacheImpl#get{TYPE}

   Region returning null in these cases, and the #determine

   {TYPE}

   AccessStrategy does not check for null. So even if this were allowed, that would not be the best approach.