By hacking.
of cource
No but you can do splitscreen.
Using different servers.
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.
Modern warfare reflex and modern warfare are the same. Modern warfare "reflex" means that its for wii. If its just modern warfare its for the other consoles
cap of 9 vs 9 on multiplayer online
No it is 3 players what are you some type of want to be call of duty nerd
No
Modern warfare 2
Well, Modern Warfare 1 on any console besides the Nintendo DS is multiplayer. But I believe you can system link play with the DS too.
If you are reffering to the cooperative missions also known as co-op then no, it has been confirmed that Modern Warfare 3 will not have a co-op component.