Two bizarre updates:
Someone else who had been ripped off by the same vendor contacted me with this complaint site.

After I posted negative feedback and was denied by PayPal (i.e. now that I have no remaining leverage over the seller; though they did shortly thereafter write a bullshit story asking me to rescind my negative feedback), I get a message from the seller saying "we've finished repairing them and have sent them back". Believe as much of that as you like - I will of course update here if I receive working parts or replacements.