I haven't looked at the grown system yet, sorry that I cannot answer your question(s).
You can try it out yourself in a copy or in a new career game.
I think this mean that player with 0-50 ovr can grow by 6, 50-60 by 4 and so on. Am I right?
I think this is not right. When you buy players from let say with ovr 70, they still can reach 78 or even higher. I think they mean the max. player growth (ovr+MAX_ANNUAL_GROWTH) for a year.
Example
Player 1 in 2013:
Overal: 60
Result in 2014:
Overal: 64
This means the max. growth! This can mean that the player reaches 61/62/63/64 or even stay at 60. You can set them higher, but it could result in non realistic player overal scores.