a) A player will get less than 50%, but the exact ratio is unknown.
b) The player will get 48 minutes for purposes of maintaining game shape. He will get the skill training as if he didn't play PF at all, and had only played 24 minutes at C. If your training position is C/PF he would train based on all 48 minutes.
Note, for that player and that week C/PF training might be more efficient. Though C/PF training is slower than C training, he would get 100% since he played 48 minutes at a training position, while he would get somewhat less than 50% of the C training.