Well, AS is moving to Terminal 6 at the end of the year which will end this bussing charade because passengers will be able to deplane through the jetway at T6 and then walk through an underground tunnel to T7 for customs.

The reason AS parks the international flights at T3 upon arrival is so they don't have to tow the planes from remote stands or TBIT over to T3, which wastes time. Also, gate space is limited at TBIT. But, it means they have to deplane through stairs since T3 has no customs or immigration facilities. I have seen them deplane through forward and aft stairs at LAX, so I'm not sure why on your flight they made everyone get off through the rear. I think normally they try to get people in F off first.

CUN-SEA gets a full dinner service, I'm not sure why CUN-LAX is still getting the snack service, but I agree that it should be dinner. Probably something to do with it being a "low yield" market. I would be disappointed if I were paying too...