It sounds like you have gotten a sweet deal! The casino hosts tie directly into the reservation system at the MGM-Mirage properties... So, you have nothing to worry about. The reservations are 'locked' at the rates which you were quoted and will not change. Travel numbers are really down and advance numbers are terrible for a lot of places right now. It looks like Mandalay Bay is stepping things up a notch to get an edge on the competition.
The flap with the MGM cancellations of reservations a few months ago was due to a large number of reservations coming in on a casino promo code after it 'got out' to the internet. The deal was meant for player's club members only and this caused issues when the public-at-large piled in on the code... They could have handled the situation better in my opinion, but it is what it is...
Since you received the offer by mail and spoke directly to a casino host, there is no question that the offer was meant for you! Mandalay Bay is a great place to stay and I have had good luck there over the years on their tables and at their slots. Check out a show at the House of Blues if you get a chance. It is always a great time!