I do not know all of the answers but let me try:

1) I do not know. SK880 does so in very powerful way.

2) disc.

3) if yes, very limited yes. You have to go to higher models like 9000PRO in order to get the multiple outputs.

4) I see my brother do that all the time on PSR740. In fact, you can control the volume of each part in real time.