Sorry to be the bearer of bad news, you're correct here. The permissions around this are blanketed to either all or none.There is a potential workaround though, you could assign the emails you want them to own to a team:
Then assign those reps to that team as well so they can only edit and publish these emails. Would that work?
These folks are already in a group. Would following your suggestion mean we'd have to go in and restrict all other existing emails to only all but that group?