1. Without knowing his full skills, I can't confirm your $16K salary estimate. However, it could be right, but that would be inflated because of his high JR. SGs with high JR have high salaries, but that doesn't necessarily make them great. I wouldn't single-position train JR, especially since it is higher than his JS.
2. Hints for training schedule. Well again it's tough to answer this without knowing his full skills, but I would keep working on his OD/JS/PA, especially passing if his passing isn't fairly high. Looking in the database, our top SG prospects with the exception of Lyle Duncan all have at least 5 passing (and Duncan's only 13th among SG prospects, not on the team at this point). Passing is also important since at least the last season or two the U21 team seems to be running a LI, in which case the guards need to be able to move the ball and distribute. I don't know what Mac's passing is, maybe he has respectable, maybe atrocious, but in either case it's an important skill.
3. Like Coco said, potential is too low for NT, but he could be a U21 player. There are a lot of good guards out there though and he faces a steep climb, and will likely have a difficult time of it. But regardless, he's young, good skills, definitely worth training even for your own team.