I had a dentist appointment in Nasha on Monday, so I dropped the computer off at the Pheasant Lane Apple Store, where they ordered a ton of parts (left and right speaker, logic board, something to do with the fan, etc.) after noticing that the Fin sensor wasn't responding. And then, mysteriously (because they said they didn't have the parts and they might take 1-3 or even more days to come in and then they'd have to open up the machine and figure out what hgappened), I got a call and email from them this morning saying it is All Ready for Pickup.
My day was sort of busy, but I canceled something and went up to get the computer, on the theory that that particular activity was scheduled so tight with when I needed to go retrieve the kiddies for their annual checkups that it would have been a serious problem if I encountered any traffic (and with Route 2 under construction, well, let's say Traffic Happens). When I got to the store, they said that the fan connection had been loose. Once tightened, everything was fine. Which, to be honest, is much more satisfying than any other possible explanation. It's not my fault, but it fits well with what I was doing when everything suddenly went to shit (I plugged in the Thunderbolt display and the headphones, and this was probably enough activity on both sides of the machine at once to jiggle a loose connection).
I am very happy to have my computer back. I am of course also happy that this was covered under the original warranty. And the next time someone complains bitterly about Apple gluing everything down in their products making them extremely difficult to repair, just say, hey, walkitout says they ought to glue _even more stuff down_.
Weirdly, my Chromebook (more about this later, altho the short form is: smaller for travel and a desire to know what the competition is like were the top two motivations) is due to arrive today or tomorrow, so if my computer had been slower to come back, I would have started using that instead.