His attitude is poor but that could easily be justified as
a) Petulance
b) A desire to win
The trouble is with Garnacho, as is the case for all of our "youngsters" is the lack of role models at the club. I fully expect Amorim (given the time and authority) to address this but for Garnacho, the damage may have already been done.
I would also say that I can't see him developing as an inside forward. He's a winger. At his best he is direct and slightly unpredictable. Technically, on the ball, he often struggles to play in tight spaces and as such, isn't the type of player you want with his back to goal or trying to thread balls into runners going beyond him.
If Amorim wants to persist with his 3-4-2-1 or whatever we want to label it as, Garnacho will need to develop as a wing back. He's got the work ethic to do it and to be quite honest, he can't be much worse than Dalot defensively however, we would just need to find a more defensive balance on the opposite side to support it.