Okay first of all I have a question.. Why would you ask us for our opinion on whether to keep or sell your player, when you have already had him on the transfer market for over a day? O.o That just seems silly to me, since you have clearly already made your decision. As for questions 2 and 3 though:
2) Obviously you should train the 19 year old 7 footer over the 22 year old, but in my personal opinion you should look at drafting a better trainee or buying one at the beginning of next season because training a star 20 year old isn't the best way to make a living. Look for a big man with at least P. Allstar potential (Or 4 potential balls in the draft) and try to base your team around that player while training him.
3) I would not do anything with your money right now and instead keep it for the beginning of next season so you can see if you drafted any good rookies. Then if you didn't, you could buy one and then use the rest of the money on your arena. Just my opinion though.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs