DDD and BBS both can be rather clunky like that... I played BBS after playing DDD though, so I'd have to replay DDD to really see about that. And considering I only have the 3DS version, I'm not about to lol. I do believe you about the bosses being a bitch though.
If by Ansem's 2nd form you mean the one where he's a big black heartless and uses the lasers, then yeah, that fight's a bitch if you're underleveled. Such was the case for me. On Normal, I grinded from around lv 35 to lv 50 before attempting him again and it was worth it - a lot easier. I was able to enjoy the following bosses with frustration that was there, but not too aggravating to make me quit for 6 months again.
Square Enix RPGs tend to be cheeseable by overlevelling, and I guess Kingdom Hearts is no exception.
Yea, to a point this is true, all depends on how the game balances the difficulty levels. In KH's case, Critical is indeed intended to have you dealing less damage.