Because you have a migration backup available for a retry, I think I'd take advantage of that luxury...
I'd probably do the 3 steps to "factory reset" the hub: Soft Reset plus Reset Zigbee Radio and Reset ZWave Radio, At this point, with those 3 steps complete, your hub is back to the way it first arrived, except the registration is still there.
Next turn off an other ZWave hubs.. such as your C-7.
Now, with nothing to interfere, can you add a device? AC powered and in the same room as the C-8 would give the best test results.
Yes, I had tried that. This comment was mainly an aside. The problem with the Tools in the Android app is that I don't see any way to determine which of multiple HE devices it will refer to. In the case I was thinking about, I was sure I was logged into the other hub.
This looks like the correct answer. Using a USB outlet was the one thing I hadn't tried before posting. I had been using a splitter on a Unify switch and could see from the UI that the power draw was <2W.
Does a higher-quality splitter tend to work?
It has been hit or miss, mostly miss but a few people have gotten it to work with Unify. For some reason it seems to mostly effect Unify, other PoE power sources are not as susceptible to the problems. It is speculated the PoE source and/or splitter might be introducing some noise on the power line that is causing problems on the hub.