i love this idea but i wish there was a simple way to play the sounds of whatever is currently selected. perhaps a play button near the top of the page or a spacebar hotkey
I added that after posting this. You can click any note on the scale, and there are play buttons on the chords. The sound will vaguely approximate the current instrument. On the Compose screen you can select the sound. I'm working on improving the sounds, this is just a first pass.