1. You should be able to, how many minutes exactly do you have? As long as you're changibng just the minutes you'll keep unlimited.
2.No they will not. The phones that are switching will be just removed from your plan. Unlimited data will continue to exist.
3.No, the contract going out will not thaffect the unlimited data at all. The only thing that changs it is if you voluntarily change it or you upgrade.