So that old backup was the only one that worked? All the others said corrupted?
2.3.8.117 was supposed to have added some checks to detect corrupted backups. There was a rash of issues like yours prior to that release and actually this is the first case I remember seeing since those fixes were put in place. Release 2.3.8 Available
Do they have a discover button? You can try pressing that. If its a main powered device it might respond so the hub can create a device for it.
You can also try creating a "virtual" device for it, and then edit the DNI to match the 2 char HEX node ID to link it to that node. Use this driver [RELEASE] Z-Wave Universal Device Scanner and try sending on/off commands which might help you find what it is. It sends Basic On/Off which just about any device that can be turned on or off will respond to. I have never seen anyone have luck making a virtual device like this but it should work in theory.
Last resort you would need to run an exclusion on the hub/device to remove the dead nodes, then include the device back again. The device should still be paired to the radio so the exclude should remove the node without leaving a ghost node.
I don't think there is a way. @gopher.ny would it be feasible to have some sort of backup integrity checker that checks the local backups on the hub and/or you can upload a backup to be checked (but it does not actually restore it)?
No, they are encrypted, only the hub itself has the encryption keys.
