N Infinite ammo and rapid fire command

Sorry, the Steam overlay is the only way you can change your in-game name. There just isn't any other way.

However, a while back, there WAS a way: Type "setinfo name (NAME)" in the console. This has however, been removed from the game and cannot be used.

Source: Looking through a bagillion sites.

Yes, there are 14 in existence. From Official TF2 wiki:

A small number of Cow Manglers were painted due to a glitch. All painted Cow Manglers were painted in Team colors, and these painted Cow Manglers had the team colors of the model and projectiles changed to match the paint.

and

October 14, 2011 [Item schema update]
-The Cow Mangler 5000 can no longer be painted.

Lots of glitched items like this exist.

I didn't think the paint actually did anything on these but apparently it does. Check out this demonstation on YouTube: