Yes you can buy them now and yes you can sorta use the room benefit.You can't log on to the AP site without an active pass and there is some debate about booking FP+ with only a voucher. Because you can't log into the AP site, you will have to go to alternate sources to find out when the AP discounts come up and then you have to call WDW to make the ressie. You will have to who the AP when you check in (although they have been know to allow you to who it a few hours after you check in if you need to go pick it up)