Powerwoman was officially a member of Luke's team, no question about that.
Can't count Skaar as an Avenger, just an ally because of the whole Dark Avengers affair.
Good point on the Secret Avengers characters. Nova might have only been around for one mission, but it was a 6 issue mission. Problem with having those long stories with dual use characters. If Nova wasn't destined to "die" he would have stuck around in Secret. Using the old Jocasta rule, think Prince and Shang have to count as allies only. I think we do need an official ruling on Sharon.
Avengers Academy is another problem. Lightspeed probably isn't anymore official than the other students. Stumped on Speedball, depends if the avenging actions of the other teachers is a reservist obligation or not.
Here's the thing about the Prince of Orphans: Cap specifically called him Nova's replacement and he worked with the team through more issues than Nova did. We almost have to count him... and I'm for counting Skaar since he joined EXACTLY the same as the Red Hulk did.