I can tell you how it worked when I used it, but there might've been some changes since then.
Yes, your Dell account needs to have the same e-mail address (back when I redeemed mine, I had to make a Dell account, but I think they've since made a change so when you join Alienware a Dell account is automatically created for you if you don't already have one under that e-mail address).
You can stack the rewards, and claim them all at once.
It used to take a while for them to show on the Dell account (up to 30 days they used to say, but mine took longer and took a bit of prodding), but supposedly that's since been automated and they should show up soon after you claim them in your reward section on Alienware.
Once the rewards show on your Dell account, you have 90 days to redeem the $ before they expire (so, if you're not in a rush, you might want to wait until closer to Black Friday or something before you claim the rewards on Alienware—just make sure you claim them early enough to have time to poke admins if they don't appear in the Dell account soon enough for the sale).
The rewards will be applied against your total when you checkout.