It doesn't sound like you have much experience with reward points. The rooms/seats/etc. are always at the discretion of the company and they usually only allow a certain amount of their product to be redeemed with reward points.

Unless the hotel is completely sold or you don't have enough points, you should be able to redeem stay anytime award, which cost an additional 50% more points to book.

Sorry, but other than the attitude you suggest was there with your discussions with Marriott (which I have never encountered), Marriott has done nothing wrong. We can be very helpful on this board if you provide us with dates/locations in perhaps finding availability.