1) As much as you don't want to hear the comic tie-in, that's the canon for why War Machine wasn't there. In the comics, Tony asks him to come and help him in New York but he's away using the War Machine suit for the Gov and it states it would take him way too long to get there in time. As a matter of fact, the last page for the comic has him joining them when they're eating at the shawarma joint.
2) I'm confused as to what you're referring too with the Captain Americab bit. When has this ever happened?
3) Simple. They already have a full cast for the Avengers movie.
If you want to make it more realistic, Loki states that Odin has to conjure a large amount of energy just to send Thor to earth. He wasn't able to send Thor and 3 of his buddies.