DMRick - Bonus Bucks coupons can be used regardless of how you book or what rate you are paying. I always book online and have used BB's many times.
Cheshire Figment - You are partially correct. The $100 BB's can be used at the hotels you listed. However, there are also $50 BB's that are good at all Marriott properties including Residence Inn, Courtyard, SpringHill Suites, Fairfield Inn, etc. These sell on Ebay for $10-$15 each. So on a 2-nite stay, you save $35-$40. Still not a bad deal.
As for using multiple coupons on a longer stay, I am absolutely positive that you can. I called Marriott customer service and they read me the Bonus Bucks policy verbatim. I don't have the exact wording, but to paraphrase it says, "It is possible for a customer to use more than one Bonus Bucks coupon if their stay is 4 or more nights. For example, 2 coupons may be redeemed on a 4 night stay. 3 coupons may be redeemed on a 6 night stay, etc." I have also done this myself with no problem.