To answer your first question, stops 70503 and 70504 actually refer to the same physical location. We use 70503 to refer to the stop when it's the final stop of a trip, and 70504 to refer to the stop when it's the first stop of the trip. Trips using either stop ID can use either platform. "Exit only" is used when the physical space represented by the stop ID cannot be reached from outside the station, which is not the case here.
To answer your second question, the stop_desc values in stops.txt are representative of how the stops are physically signed in the stations, in order to assist riders with wayfinding. Once the Medford/Tufts branch of GLX opens later this year, the D branch will begin going to Union Square and the E branch will terminate at Medford/Tufts. The station signage has been designed to describe this eventual state.