1) Yes they do. They should let you know which days those are or you can call to inquire.
2) to be honest I am not sure. I would imagine that the next time you have to pay your deposit, you should narrow it down.
3) you said a nice email to the admissions people stating that you are grateful for the opportunity, but are declining the acceptance so that it may go to another deserving student. There are templates on here if you search.