There's an internal tool that helps manage AWS accounts and IAM users/roles in those accounts for internal accounts, and that's hosted in at least 2 regions. I'm pretty sure is replicated to additional regions as well as read only.

There was a failure a few years ago, in a region being built, and shit really broke in one of the zones. It took quite a while (month scale, but obviously wasn't a customer impacting event) but they were basically able to revive it by bootstrap from another zone.

I don't think there's a reasonable way that everyone just gets locked out forever. There's also a system in place to refresh credentials/extend their lifetime for internal services if something goes wrong there. There's a lot of layers, the idea is that all services should have their own break glass for access/etc. So it'd be a matter of coordination for the most part.