Update on our sometimes-flustered '08 Honda Civic!
You guys nailed it.
Official diagnosis: the
starter is wearing out and about to give up. They also confirmed the battery and alternator are both in good shape. After some driving around and several restarts, they even managed to reproduce the failed start that I could never get to happen myself.
It's an expensive repair through the shop, and I
could save $500 by ordering the part and installing it myself, but for peace of mind and to confidently move past this issue, we're just throwing a fat stack of cash at the shop to take care of everything.