Not sure I understand your new questions properly sorry, but this is my best effort based on what I think you mean:
If you replicate the variable you would need to adjust things, Prone_Server should obviously not call Prone_Multi, and should instead just set the variable. Then instead of Prone_Multi, you would do the Is Prone check and montage playing in the repnotify function, but with montages swapped around (because you are reading the just updated Is Prone now). Of course, drop the delay and sets.
The variable needs to change on the server to be replicated, yes. So the Server RPC is essential, replication is a one way street (Server → Clients), getting the server to do something a client wants always needs an RPC.
The point Auran just raised is the ‘killer application’ here I think, by doing this with a multicast anyone who comes along later will see you prone character standing, because they never got the multicast; using replication the engine will take care of all (in this case, replicating the changed values to the newcomer immediately).