I'm a little confused about your first question, but it is true that Brown wants to accept students who want to attend Brown first. If you indicate that you would not go to Brown if not accepted to PLME, it hurts your chances of being accepted.
As for ED acceptance rates -- most athletic recruits are accepted ED. Their chance of acceptance is 100%. If you take those 200+ acceptances out of the equation, and also remember that many legacies apply early, the ED acceptance rate drops. Even then, it's not as low as the RD rate.
Since it's pretty hard to do a blind study comparing the results of identical students who apply ED and RD, there's really no way to answer your question. There are fewer applicants applying ED, so your application might get more attention. I'd rather be the first tuba-playing environmentalist's application read than the 1000th.