No. He had nothing to do with The Avengers(616), but in The Ultimates (Ultimate Avengers) he put the group together and one time unisciti them in battle against the ultimate skrulls.
Captain America is the leader of The Avengers, but its anyones opinion.
No. Nick is not an Avenger, but he is the leader of S.H.I.E.L.D. I do not believe the Avengers have a leader, unless te count Rogers because he (technically) was the First Avenger. That doesn't mean Stark will listen to him, though...
No he is not and i wouldnt really think of Captain America as the leader of the Avengers. he may hgave gotten the title, but i think Iron Man was really the leader.