No, there's no reason we wouldn't allow necromancy as it exists in Thedas—we have several necromancers in the game already! As for the holes, size doesn't matter. We just need one version to be fully accepted as your canon and the others to be supplemental. So, for example, you could say you're using 3e but supplementing with 2.5 mechanics where 3e doesn't have them, but you can't discard 3e mechanics that do exist because you like 2.5 better.