Fixing a forest that can't talk to it's top level DC's

We have a client that is running a forest for no known/fathomable reason. They've got several subdomains, and anyone that knows why is long gone.

All of the user accounts exist in a subdomain. And much of what happens on the network happens at lower levels as well.

At some point in the last 6mo or so, the DC's at the very top of the forest seem to have stopped communicating, without anyone noticing. Possibly as a result of them deciding they were on a public, rather than private network. Or possibly as a result of a major disaster involving loss of the storage system underpinning the VM infra 6mo or so ago. This hasn't actually caused anything to stop working in a way that users notice.

If I'm on the DC for the top of the forest, and run repladmin /replsum, dc01/02 both report "could not find the domain controller for this domain". Even tho that's themselves, first and foremost. Largest delta is >60 days. And reporting 22 of 30 for 73% and 17 of 25 for 68%.

Schema Master and Domain Naming Master are both on DC02 from the top of the forest, and thus also messed up. The other roles are on a DC serving a domain that's operating just fine for now.

So is there a way to salvage this mess, if they DC's at the top are tombstoned?