All doable. Can be tricky though.
The sound desk output will be mixed for the system, but in the circumstances is probably fine. Audience sound is important. Syncing - there is software to do it. Make sure all the cameras are on the same frame-rate!
Mobius cameras are a great cheap, light way of capturing footage like this - stick one on the keyboard, one on a guitar neck... The wide angle one would be better for a keyboard.