So, despite not being there and not interacting with the passenger like the CC did, you’ve come to the conclusion that the investigation was ‘cursory and ineffective’? Impressive!

‘In reality’ his condition was such that you have no idea what his condition was since you weren’t there. The OP indicated that he was asleep. I also find it difficult to respond when I’m asleep.

BA rules state that one must be physically fit and able to assist in the unlikely event of an emergency and that they must be willing to assist, and that they are able to read and understand English. I haven’t read anything in the OP’s account to suggest that he didn’t fulfil these conditions. It doesn’t say you cannot drink and it doesn’t say that you cannot be asleep. He was apparently elderly, but that’s such a relative term as to be almost meaningless and when pressed, the OP still couldn’t give an idea of how old he was.

There was also also nothing to suggest that he was unable to walk off the plane at the end of the flight. On the basis that that occurred only a few minutes after the landing (the critical time according to the OP) it’s reasonable to assume that he’d be able to get out of the exit in an emergency a couple of minutes earlier. I imagine in the event of an emergency he’d probably wake up fairly promptly. Of course I wasn’t there so I don’t know for sure.