Ya your right, SF are the hardest to train and thats why they sell the highest on the transfer market.
Secondly your correct again, but thats the way is game is built. You can't focus on two players in different position unlike in real life. If you were to train and center one week and a guard in the next week then both players miss training and not reaching their full potential. If you want to get the most from a player then focus on just them. You can get a second player to train in that position too.
Hope that helpful and sorry in the delay of my reply