If he plays 23 minutes at PF, and 25 minutes at C, he won't reaped the benefits of training because he didn't receive 48 minutes at C, right?
Correct, he will only receive 25mins worth of RB training for that week. It does not equate to 25/48 of maximum training for that week though, it will be a bit less than that.
That means, it's best to train C/PF so that he receives the full 48 minutes. Am I correct?
This is up to you. Training single positions will get the maximum results in a shorter time, but obviously it limits the number of players that can be trained effectively in any given week. Training more allows you to train more players, but the speed of training slows a bit.
Somehow he plays 46 minutes at PG and 37 minutes at SF, he still wouldn't receive the full training in Passing because I selected PG only and he only played 46 minutes as a PG even though he played a total of 83 minutes for that week.
Again correct, he will receive 46mins worth of P training, which is again slightly less that 46/48 of max training for that week.
Hope that helps.