Glad to hear you're going to see the vet, thats a definite must.

The death of the the other cat may be a clue, possibly. If he got on well with it, maybe the other family were taking him in, and he was getting al his cuddles there. Maybe they turned him away when their cat died. Just a thought, I wouldn't recommend approaching the other family with that suggestion!

Providing al is physically well, it's great to hear about a personality giving rise to a more affectionate cat. It's usually the other way round (or maybe we normally only hear about it when it's the other way round?...)

The nervousness followed a quick switch recognition is also very interesting...I have no idea what it might mean, but it's interesting!

Keep us up to date with progress, it's quite intriguing!