Update: He is upright! He is on the same piece of décor that he'd flopped onto yesterday, but now he is upright and moving a bit. His right pupil still looks cloudy (it almost looks like a cataract), so I think there's definitely some damage (neurological? stroke?) but he is able to hold himself upright, which is a very hopeful sign. Tomorrow, I may try to tong-feed him, just in case he needs a little help on that front. (He was immobile/unconscious/sleeping through last night's feeding, and if he's unwell, he may need some assistance for the short/long term.)
I would still appreciate any advice on nursing him to health or explanation of what's going on (medically) per the questions in the OP above, if anyone has insight to share.