Using different servers.
No there is not.
By hacking.
of cource
xp on Private match's
No but you can do splitscreen.
no
play in a private match
go to private match.
Some times.
2 players in campaign are better.
Yes, there are many invisible, godmode, aimbot, etc players found on Modern Warfare 2. Some even have the ability to infect other players, in turn giving them the mods as well.
If you are the player hosting the match you can close the match, but if you are not hosting you can only stay or leave the match.