First of all, it's great that you decided to join BB. Welcome! (I used to play HT a few years back, but that is neither here nor there haha).
1) The training is a bit weird, especially when compared to real life. For Passing, you want to play the player you desire to train at PG and he should play at least 48 minutes at PG that week, for example. He could play more if you want to, but 48 minutes at a position is all you need for full training.
If you are training Jump Shot, Rebounding, or One on One, there are 2 positions that are trained instead of one. However, 2-position training has 70% of the training speed that 1-position training has.
2) Yes, your players must play 48 minutes a week at that position you want them to be trained at. You'll know they'll get the full training possible when you look at the "Training" area and see the player's name with a 48+ next to it.
3) Looking at your team, I would train Glinnuer, Taras and Ebedli. Even though the latter two guys are 1 season older, they have more potential and can be trained to a higher skill set than Glinnuer. I would make Glinnuer the third trainee. Try to get him 48 minutes a week along with Taras and Ebedli, but don't sweat it if he doesn't.
You can sell your players for a profit, or you can keep them for your team if you like. Nowadays I'm keeping my players for myself because for me, it's not easy to get the player I want for a cheap price. You don't have to abandon the rest of your team. If you like, try to win as well. It's not always convenient or easy, but it may add to the fun.
4) The age you should stop training your players depends on your patience, the potential of your players, and how old they are. I or someone else can go into detail about this later.