You can think of height already being factored into a player skill, a 7'4 player who has just learned to tie his shoe laces might have the same rebounding skill as a 5'4 guy who has considerable leaping ability, timing, upper body strength, and positioning - but these simply make up for the 30 inch difference in height plus arm length. The game engine can simply make it a 50-50 contest to see who gets the rebound, without trying to make an adjustment for height.
The taller player can improve his rebounding quicker - for example, he can learn to raise his arms over his head, while the shorter player is already closer to his physical limits.
The best position is calculated, and can change as the result of training. The formula appears to be based somewhat on relative skills, so that for example a "point guard" has better passing and handling skills relative to his shooting. If you want to see a very bad player, search on the transfer list for a "point guard" with a maximum of inept passing. His shooting will be even worse.
The best position has no effect on game performance, which is based instead on the position a player is at, and his actual skills (along with tactics and the opposing players). A player can play multiple positions in a game (I've had one player play 4 positions). In addition, a player may take different roles on each trip down the court, with a center sometimes shooting from 3, or a guard driving the lane. Whether the center hits the 3 depends on his shooting skills, not that he is "center".
The "best position" appears to be taken into account when calculating the salary. I'm not sure if there is a way to manipulate skills to take advantage of that or not.
The staff system is going to be changed in a few weeks.
I'd certainly hire a better trainer, once you've decided on what you are going to train and have the players to train.
A doctor will not help injuries heal, but will reduce their severity when they do occur (longer injuries are less frequent, and many injuries will only last for the remainder of the game). He might be considered more of an insurance policy, and it might depend if you have anything worth insuring. It could be cheaper to have a couple of extra backups than to hire a top notch doctor.
The effect of the PR guy is somewhat unknown. The BB people insist that it makes a difference in attendance - but it is hard to pin down the effect, since you would have to have two comparable teams with different level PR guys.
You probably can't afford level-10 staff members.