it varies by school. if the school requires a pre-MS01 summer research rotation beginning in July, then they can set their deadline to april 15th (Cornell).
and yes you can accept an offer while still holding your waitlist spot and then withdraw your acceptance if you get off the waitlist. you might have to pay a deposit with your acceptance of the first school, but more likely than not you'll get it back when you later withdraw. schools know what the deal is and this happens often enough that you neednt sweat it.
HTH
