2) No. Choice Band only boosts physical attacks. There is a new item in Diamond/Pearl (Choice Scarf, I think) that is the Special equivalent to the Choice Band.
Actually the Choice Specs. Choice Scarf is for Speed.
It does lock you into a single move, but allows you to more damage. And because people will often switch to a pokemon that can take physical hits better, CB allows you to deal more damage (for example someone switched to a Swampert on my CBmence, evidently to try to Ice Beam, but lost nearly half its health)
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