They bought you off SUPER cheap. In fact, I'd say they ripped you off twice. First when they walked you and second, when they lowballed you on the compensation. To answer your questions,
1. A reservation is worth $200 + 2 free nights + costs of inconvenience + alternative accommodations. See below.
2. You have the right to the room, but they can pay you the above and absolve themselves of liability.
3. It hasn't happened to me yet, but if it did, I would not accept anything less than what is in #1.
If you are at least a bronze in the Marriott Rewards program, you should get -
The Ultimate Reservation Guarantee: To claim, the confirmation number for a reservation guaranteed with a major credit card must be presented, and the Marriott Rewards membership number must appear as part of the reservation. The specific room type requested will be provided when available, but not guaranteed. If a room cannot be provided, the hotel will pay for your stay at a nearby comparable hotel and the member will be reimbursed for the inconvenience. Compensation varies by lodging brand (see below):
For Marriott Hotels, Resorts and Suites; Renaissance Hotels Resorts and Suites and Marriott Conference Centers: Alternative accommodations, plus $200 and 2 free room certificates to guest.
For Courtyard, Fairfield Inn, SpringHill Suites, Residence Inn and TownePlace Suites: Alternative accommodations, plus $100. (Platinum members also receive 2 free room certificates)
I would follow up on this. They screwed you.