1. Two units per base, so four total if you have Mother Base + one FOB. You have to pay real money (build more FOBs) to get the capability to send more units out at once.
2. That should not be possible, maybe a glitch because servers were offline/out of sync?
3. Just keep them around for when you expand your platforms and have more staff capacity, there's no penalty for having a full waiting room. The AI will automatically get rid of the worst ones once the waiting room exceeds 355 people (not sure if that number is fixed, but it's what I have).
4. The economy is really harshly balanced in this game, trying to push you towards buying more FOBs (which means more combat deployment teams and faster material processing). Your only choice without investing real money is to spend a lot of time farming/grinding materials (especially fuel).
5. No one will walk away (except if your GMP's in the red, which it should never be at any point), you'll get better and better recruits as you play, so I wouldn't bother contracting any one of them directly. What's a great soldier early in the game is just average later and garbage endgame.
6. "Boaster" is a skill that leads to a player's binocular analysis being off-base. It has nothing to do with the lock. I'm actually not sure why some seemingly random characters are locked like that, I have one or two of those as well. There doesn't seem to be anything special about their skills or anything... If anyone knows more about that, I'd like to know, too.