1) Like Mashbar said "Yes - for as long as you remain a paying customer."
2) You will need to set the Name servers to Dreamhosts in order for them to work.
3) Mirrored, and redirected sites dont count toward this total. So if you have downloads.mysite.com just redirecting to www.mysite.com/downloads it doesnt count towards your 15.
Dont forget Promo code 777. to use it sign up for a year prepaid of CDI, (No setup fee). Choose your new domain name.. and when it asks, use promo code 777. your total for the year (Including Domain name Registration) will be UNDER $10 (77 cents a month).