Hi, for your bullets:
If you set them to replicate, then spawn them on the server only, not on the clients. Because they are replicated once the server spawns them they will also exist on all the clients.
Your problem is that you set them to replicate and spawn them on server and clients, therefore the server will only see the bullet he has spawned, but the client will see the bullet the server has spawned and the bullet he has spawned.
For your health bar:
Can you show the code, where you set the health bar percentage?
As for why there is only one client spawned: no idea.
But you can try to spawn them manually and debug from there, so set the default pawn class to none, and then in your player controller on event begin play, you can spawn and possess your pawn on the server.