1. Skills that can be trained at two positions (like RB) require you go give the player 48+ mins (45+ for 18/19 y.o.) in either of the two positions in order to get full training. If we take RB as an example - the trainee could play 48 mins as a PF
or 48 mins as a C
or 30 mins @ PF + 18 mins @ C. Doesn't really matter as long as the combined in the two positions is over 48.
2. Yep, that's right. Low is x.00-x.33, medium is x.34-x.66, high is x.67-x.99.
3. Yep again. Game shape is a vital part of the game (as it is irl). DMI stands for
Deliberately
Meaningless
Index.

It is not really relevant in terms of player quality because it mostly represents main skills values and also includes FT as well. It could be utilized to calculate the exact levels of the player's game shape (e.g. x.42) however, as long as you know what it was in the beginning of the season (that's the only time when we know for sure that the GS is exactly 7.00.
4. The main thing is to make sure your players get 48-72 minutes playing time in the three "main" games every week. Those are the two league games + the cup/scrimmage game. Some of the staff could help you with it too. The massage doctor lets your players stay on the court for a bit longer than those 72 mins without being penalized (high stamina helps with that too) and the psychologist boost game shape greatly too. The latter is deemed to be somewhat OP actually.