My MIT ID# was on my offer letter, but it took about 2 and a half weeks from the time I submitted my reply form to the time I got to create my Kerberos account.
In re: to housing, I've also heard that Tang is fairly undesirable, if only because it is pretty far from the campus and grocery stores, but I also hear that it has great views of the Charles, if that is important to you haha. For the selection/ranking process, it allowed me to rank all of them, you're not restricted to your top 3 to rank. You can rank all of the housing options if you want. Grad students I talked to, though, cautioned against giving a ranking at all for places you would absolutely not want.
I'm banking on getting into campus housing for the first year to avoid the hassle of finding a place off-campus, and then the second year may try to get a place off-campus or try to move into the Eastgate family housing with my SO.