When David is playing the piano at the end, he is sitting in the wheelchair with his left side toward the piano (playing with his left hand). From the sound, the piece he is playing makes use of the piano pedals, specifically the damper, but the way he is sitting would make it impossible for him to get his uninjured right leg to the damper pedal to use it.
Hawkeye's remark about "seafood mama" is a reference to the 1956 Fats Waller song, "Hold Tight", released three years after the war ended.
At first the shadow of the boom mic is visible on the ceiling of the mess tent, and then the boom itself is visible as BJ stands and begins talking to Klinger. DVD time 10:25
Klinger's footlocker wouldn't carry nearly enough sand to make the picnic "area" look or feel anything like a beach.