The MultiplayerSpawner works by you telling it what nodes it should watch for, and pointing it to the spot in the scene tree it should watch for them. When the authoritative client adds a node to the tree where the spawner is watching, the spawner will automatically handle creating that node on the other clients.
If you want to use the MultiplayerSpawner, you should not be manually spawning nodes anywhere but the authoritative instance.
Synchronizing player data still needs to happen (it isn't handled by the spawner). You can do this by RPC as well, but it can be easier to set up using the MultiplayerSynchronizer (CAN be, not always).
What I was trying to say regarding sync is that I don't believe you can send nodes over the wire with RPC or MultiplayerSynchronizer (maybe I am mistaken, though)