Huge links on a draining day:
After getting the DNS tests to pass, I did a topology rediscovery on the troublesome DC. It came up with a new replication link with a funky GUID, and event logs all over the place indicated that AD said, “Oh crap, we had the wrong GUID for the PDC emulator all along!”
At least it (NTDS Replication) appears to work now. Off to class.