That's kind of the point, though. Everyone does their own thing, and then the listener has the choice of which one or ones to play at any point during the song.
No. There'd be a reference track (drums or something) as a starting point, but after that the players are on their own to do whatever, so long as the track is a fixed length. Then there'd be an interface that lets the listener play all the tracks at the same time, or just play some of them, or play them all but have the individual tracks fade in or out at points that the listener defines.