Closing this, as we've finally gotten at least some response, even if it's dismal

Well, we finally have an official update, which is all I was asking for. Even if it's...depressing.

https://us.forums.blizzard.com/en/wow/t/guild-bank-missing-items-update/1963451

Doubt any further attempts at remediation will do anything at this point. They screwed up, and admitted that they can't restore basically the majority of what was lost.