Just recently there was a thread on the LokSound group (on groups.io about some ESU decoders dying). Not specifcally nanos, but they were all V5. I also seem to recall that few months back, someone had a batch of new V5 nanos dying not long after purchase. There might be a bad batch of components ESU is using on their decoders.
As for ESU tech support, what can they say over the phone? There is no magic command they can give you to revive a half-dead decoder. You already tried a reset. Not much else can be done.
I assume of course that you tried to successfully load a sound project to another V5 ESU decoder (to verify that your LokProgrammer setup is not the problem. If you haven't yet, that would be a good thing to try.
You also mentioned that the decoder failed on the decoder tester. Is you tester connected to LokProgrammer, or to your PowerCab? If to the LokProgrammer, then again test your LokProgrammer to see if it will work with other V5 decoders. Yes you mentioned that the first decoder worked ok, but maybe the LokProgrammer died after that decoder was done. Just sayin' . . .
Those are my off-the-wall ideas.