Firsty, hello and welcome
Ok to try and help...
1) For training you cannot have any friendly planes helping you unfortunately, its just you versus the AI. Sorry bud.
2) Beating the campaign will unlock new planes and load outs for you which you can use in training mode.
3) For which planes are unlocked and how then this will help, its a guide of every in game plane and whats needed to unlock them -
http://forum.1cpublishing.eu/showthread.php?t=13167
4) Arcade mode pretty much makes everyone on a level playing field no matter what plane your flying. Alot of players prefer this mode for the franctic action it offers. Realistic and Sim make things moe difficult, with Sim mode its cockpit view only and limited fuel/ammo. Its all about finding the one that you prefer.
5) Mission editor? I'm not sure what this is. As for actual missions... no you cannot change anything in these other than difficulty.
6) In campaign certain requirements are required to unlock planes. If you check the link i've given you then it will help expalin whats needed.
Theirs "single missions" in the game aswell. Playthrough these to unlock additional planes. Again, the link will tell you whats required.
If you need anymore help then post away and we will all be happy to help where we can.