True but it's a POS stadium with little to no commerce in the surrounding areas. You are right it's an option but it falls short when compared to Nats park.
The area around Nats Park has blown up lately as well with quite a few more food and drink options opening up after the baseball season ended. Having said that, a lack of commerce in the surrounding area doesn't seem to faze the NHL. Heinz Field in Pittsburgh has nothing around it.
To me, the football stadium layout is much more conducive to a hockey rink though. The outfield seats in a baseball park are worthless.
The problem with FedEx would be the possibility, while admittedly remote, of a home playoff game being played two or three days later (something the schedule makers can't help with). This depends on the schedule for next season of course, but the wildcard weekend always seems to be held on the first weekend in January, which would be the 3rd and 4th next year.
RFK or Nats Park make the most sense. I like the idea of shaking the RFK stands though.
Edit: Forgot to add, Nats Park more or less no longer has sight lines to the Capitol due to the Apartment Building construction to its north.