About Q #3 … It is actually possible, I believe there has been said that the host of a server can decide which mods are allowed in the server. So even if you use the op super weapon mod. Doesn´t mean you can use it in someone else his/her server.
Can I create a mod, which gives ME a super weapon to bash my friends face in?
i.e. can I use mods to create an unfair advantage?
If you create a super weapon mod and load it on your server, all the other players on your server will sync to that mod. If everyone has a super weapon, its not super anymore. In which case I stand by my answer!