Net Play
 

The LF on Team R comes down from an attack with his/her foot partially across the center line. As the player tries to return to the LF position at the net, a Team S player comes down on his/her foot. This causes the (a) LF of Team R to fall into the net, (b) Team S player to fall into the net. What's the call for (a) and (b)?

It seems like you're implying that the foot-on-foot contact occurred completely on Team S's side of the net. My answer is based on that assumption...

(b) is more straigforward. In every rule set, this would likely be judged as a centerline violation on the Team R player for being on Team S's court and causing the player to fall (interference).

(a) depends on the rule set. In NFHS, it's a net violation. In NCAA and now USAV (new rule), it's a play on as the Team S player wasn't playing the ball when they hit the net.

This is not a center line violation unless there is interference with a player playing the ball.