1) i didnt get why he launched the nuke either..
2) cant remember why but im pretty sure its explained in the game
3) nothing happens to makarov, he helps you find general shepard when your in the airoplain graveyard.
4) and yes ofcourse there will be a mw3
The Nuke helps the americans. When you play Whiskey Hotel level, you and your squad are being surrounded by a lot of russian helicopters. Then the nuke goes off in space and creates an EMP effect. All the russian helicopters and tanks go offline. Thats how you and your squad survive.
General Sheperd betrays you because he hired Makarhov to attack the airport and make it look like the americans did it. Sheperd then gets a blank check from the american government so he can win the war. Sheperd just did it to look like an American Hero.