I feel for you, even a little moodiness is worrisome once your cat had bladder problems.
The way you describe him wolfing down his food and being snappy with his siblings made me think that he was involved in some territorial fighting, and that he might had spotted his enemy.

his undercarriage bits didn't seem unusually large and his 'swingy' flap seemed unaffected.

LOL, is that what you call it in Scotland?