I've heard that Facebook doesn't actually hard-delete any information, so odds are that when support gets back to you sometime this year, they'll have admin tools that will allow them to reactivate the group.
I don't think this is an ability Facebook support has. Engineering could probably do it, but I doubt you're going to be able to get this issue on their radar.
Keeping the data for European users after deletion would be in breach of GDPR, so Facebook does delete data after some grace period. It doesn't necessarily mean physically removing files, but you can do other things like storing data at rest encrypted with with an account unique key and then removing the key when account is deleted.
(both what you say, and that Facebook support actually gets back to me sometime this year!)