The 'clear existing tags' setting is irrelevant here, because I'm only concerned with tags in the 'preserve tags from being cleared or overwritten' box, which itself overrides 'clear existing tags'.
I can solve the problem of tags being cleared by getting rid of the $unset scripts (which previously removed MusicBrainz values without affecting the existing values). The problem I can't solve is of empty tags being set with values I don't want. For instance, if Label (aka Publisher) is empty, I want it to STAY empty. There's no way to achieve that without using an $unset script, which in turn removes existing values.
So either I can preserve the existing tags, or I can keep empty tags from being set, but not both at once.