Can I use an Air-mobile point to make a unit air-mobile for retreat?
Yes. In fact, it's the only way an attacking unit without a pursuit modifier can continue to pursue, so it can definitely be used as a retreat tactic.
Do 1st Air Cav and 101st units retreat using Airmobile movement point costs?
Current situation I had a HQ uncovered and VC attack it but don't "kill
it, I want to get "far away" from them but Mechanized movement won't get me far enough away. Can I spend an Air-mobile point to let it be Air-mobilized for the the retreat move?
1AC and 101 can always use it, but they don't have to. The only restriction to who can use airmobile are the ones with a shaded (white) movement point. Those are ground movement only.