I'm sure I saw someone say words to the effect of you can't learn composure but I disagree. You can work on controlling your emotions and also on the things that effect your emotions; if you want to. You can improve your reading of the game. I know from the many years that I played in goals at the very lowest of levels (i.e. recreational) that my positional play improved with experience - I never had any coaching but I never stopped trying to learn.
As you say, there are certain players that he is made to mark (Costa is another oft quoted example).
As for the incident you mention in the Hammers game, Ederson should have pumped that one down the park but receiving the ball under pressure and dealing with the situation is something you can practice over and over in training. The turn and spin Mangala did in a prior game shows that he has skills.
Like you, I'd rather just go and buy VVD but my faith in Pep means that I think he can get the best out of Mangala.