The way I see it is you have gone for a heap of young players to train a national team player. On a quick glance at your list, you only have one player who can get there. To get him there will not take a season or 2, but you will have to commit to single position training for at least 4 to 5 seasons. This will therefore diminish the development of the other young players you have bought.
You could train multi positions to evenly spread your growth, but what you will end up with is a balanced squad with no real match winning players. And also with training youth you have to look at how much it costs. My player who was U21s last year had a start of salary of $5200. With an exceptional (lv 6) trainer costing me on average around 50-60k a week, 4 seasons later he has a salary of 59k. It is incredibly hard to develop a team whilst developing a player of that level in lower leagues due to financial strains. I got demoted this season and have lost over $100k a week. I still make profit, but if I dont get promoted again in the next 2 seasons I need to cut players.
I can understand selling players because you believe you can get a good price, but the reality is they have to be replaced. By selling a team that got you promoted and replacing them with youth was always going to weaken your team in the short term, meaning almost certain relegation. I had my star that I trained and had a second trainee to develop similar skills. When the back up reached a certain level, I would sell and replace with another youth and an older player with much better skills in a different position. Now my star SG has capped at 22, I am training talls with the same philosophy knowing that I can get his optimum level by the time my other star is still mid 20s, and still fill the rest of my lineup on the transfer list. In saying that, this it has taken 4 seasons to make this a possibility, and will take another 4 or 5 to achieve it.
To be honest, selling your players for youth is going to set you back a long way. When you get promoted, you have to instantly strengthen your list just to stay afloat. Now it will take seasons for you to get back. If you know relegation is coming, I would train 2 players in single position and sell both at the end of the season. Then buy key older players to fill starting positions and have 2 18 year olds training in the position you want your NT prospect. Keeping in mind you will lose out on the youth you have and your income will drop next year...
If anything, at least you will be a better manager for working through it whatever you decide to do.