It's true he is a bit of a mental midget but at least, he has obviously a big potential and loads of talent.
You can always improve your mental toughness (see Djoker) but you cannot improve your talent.
Good point, but it's also true that a player of lesser natural talent might consistently outperform more talented players, who can't perform to the best of their abilities at crucial points in a match, because of mental frailties.
I like to think that Dimitrov will improve his mental stability and he certainly has time to do that, but he is almost 22 now. If he doesn't break through this season or next there might well be the added pressure of he himself starting to doubt he will make it big.
Anyway, this is all speculation now. You are definitely correct that he has the most natural talent out of the bunch.