In 13w02a you can name mobs: I was wondering if you'd be able to rename players without changing/breaking their skin. (Not sure if this is in the right section)
You can with Packet20NamedEntitySpawn, but I recommend using mbaxter 's TagAPI. Renaming player's nameplates will change their skin according to their nameplate, but if you are only using ChatColors then it will keep the player's skin.
You can, if you only add color. But otherwise, no because the client generates the skin based upon the name tag.
Oh, I don't know about with 1.5 you might have to wait until it comes out, or when Bukkit 1.5 comes out.