I don't think United will be active in the January transfer window because it will be more costly to get the right players during this period. Also, most players prefer moving in the summer window, and the kind of players available in January window are fringed players with less game time at their clubs. Two areas that I think needs reinforcement are left wingback and a top striker. They've signed Diego Leon which will be available next season, so Amorim just has to make do with his crop of players till the season ends and bring whoever he wants in the summer window.
Definitely! Imagine I'm a manager that is having a great times and have chance of winning the league and you said to me that you want to buy one of my best midfielder, I wouldn't want to jeopardize my potential trophy for your own formation. If you are going to buy the player the amount I can make from the trophy, you can take it and go. Deep down, they will not sell to you. Maybe a team that are in between top of the league and the relegation might sell.
Amorim should just focus on try and error system, there is one way formation that can make this players becomes the best of their self. It's not going to be easy because there is going to be losses and there maybe win and it's not guaranteed but it will work. All the teams can't be that bad and that can even help him understand people that are not ready to contribute to the teams and also selll some to have more money if they sell some of the players.