A zoom from the back of the hall cannot capture the best sound of the piano. The distance will compress the sound and make it mushy. Your pp was substantially lost, and your fortissimo was pretty much mezzo forte. If you want to get the best performance out of the little Zoom, try hanging it from a microphone pole near the strings. Then have someone record the video on a separate camera. You can blend the two in any standard video editing program. Better yet, borrow two good condenser microphones for the audio and match it with HD video. The HD video is easy these days. Just about any good SLR camera comes with HD video mode.
Here is a sample of the video quality of such a camera (Canon SLR) Sample HD video
Even after youtube compression it is pretty high quality video.
The sound is coming from the SLR's internal microphone, and the camera was at the back of the hall, so the sound suffers from the same dynamic compression as your Zoom. Still, it's not bad at all for an all-in-one device.
If you have friends with SLR cameras that have HD video capacity, you could create a very professional set of videos that have multiple camera angles. If you don't enjoy producing edited videos, again, you could find a friend who does. A few videos of this sort would really show off your considerable talents!