1. I believe Captain has nothing to do with performance. If someone makes a test, playing with the same team against the same opponent, with the same conditions, ten times (friendly games) and having as captain the best player (in 5 games) or having as captain a medium player (the other 5 times) and show the results, then I 'll be convinced that captain does matter.
Everybody just put as a captain an old player, a high q player or a favorite player.
2-3. Use of foot or height. I never noticed something that affect the behavior of my players. They are cosmetics.
4. I believe that also doesn't make any difference. I had a free-kick specialist with both feet and I was reading some comments that it wasn't his favorite side . Also had another player and in the same season some comments were talking about his favorit position while others saying the opposite

ps : there is about 1 M' players, with 20 players each team + players from TM. If every player had special characteristics, the program would need ages to log and run.