If there are less then I think it’s six players, you don’t get the full credit amount. If you are only two, you gain almost nothing. It’s to prevent people from farming on empty servers without really playing the game.
Yes, you gain less credits if teams are grossly uneven (like one team being two men or more down) not sure how much though.
I agree, would be nice. The reason credit gain rates are only marginal based on xp-gain (‘skill’,) is because it’s very susceptible to abuse. You could easily imagine a couple of friends getting together on a empty server as engineers, planting and disarming each others charges. Which is just nonsense, they should be playing the game, not abuse a mechanic to farm credits.
The other reason is, by instead giving a winning bonus, you emphasize teamwork. It’s not your individual skills that are important, it’s the team skills.
There isn’t a good way to farm credits. You just play. If you have at least a certain amount of players on each side for the full match you get pretty much the full payout. How you play and if you win only adds a small % extra.
Just play to win, do the daily missions, make sure you do at least one objective and one stopwatch and win one of those and if you feel like it then buy a booster.
By far the most important are the missions, so be smart about it. Try to complete a 3 hour set whenever you get a chance (i.e. playing for a bit every three hours is more efficient than playing three hours in one go). When you stop playing or the refresh is soon, discard missions which are not efficient for you. Of course in some cases this is like poker, where you have to work out the probabilities of getting a better hand before discarding a card. 
Also, if you don’t have time to play, sign in and dismiss any missions you aren’t interested in and log off. You will hopefully get one that you prefer while you are away. I try to do this before I leave for work to increase my odds of getting missions I like.