I'm not an expert on little Johnnie, but I know that he has done several questionable things. Didn't show for the Manning's camp, taking money for autographs, etc..
Just the whole concept of being a celebratory QB, especially from the very beginning, can lead to distractions. That's why Parcells didn't want a celebrity QB.
Again what you are saying is fair, but not sure that is mental makeup.
That sounds an awful lot like maturity and guys like Favre and Rodgers had those issues as well.
For me mental make up is:
Do you love football?
Are you a self-motivated person?
Do you want to be the best?
Can you learn the intricacies of the game and win mental chess matches?
So for me, he is fine mentally if not in maturity.
And those are not at all the same.
A lot of guys grow up and mature but also lose love for the game and a desire to be the best.
JM truly wants to be the best and has confidence that he will be.