o Any room for significant doubt in the diagnosis above?
Nope. That hard drive has bit the dust.
o I left the MB sleeping in a rather warm room, maybe 90F/32C. Might this have had anything to do with the failure?
o Any reason the Apple Hardware Test would not boot? Any reason to pursue this? I've heard that AHT doesn't really say much in the case of a HD failure.
This is disturbing. AHT should boot on the machine even with a mal- or non-functioning hard drive.
o Would SMARTReporter (or a similar product, if one exists) likely have warned of an impending failure?
Maybe. No way to know really until the exact cause of failure is known.
I would replace the hard drive. Very likely you will be able to get plenty of use out of the MacBook even if the optical drive is not working correctly.
Best of luck.