The answer to all of the above questions is the same - it is "he could be". He has the attributes to become great at all of these things. Certainly right now, he is better suited in a Lampard type role which is where i would expect Jose would use him should we get him. However, physically he is capable of playing deeper and breaking up play. His passing ability is actually fantastic.
From a fan's perspective, you are getting someone who is supremely talented and can already perform on the big stage. From the club's perspective, you are getting someone who is young, knows the club, has bags of potential to improve.....and huge commercial potential. Sadly, that last part comes into it.
I agree with parts of your post. Lampard, Toure, etc., that is exactly the kind of player I see him become, may be a better version of them. Someone who is going to burst from the midfield and get us 10+ goals. I actually think he'll replace Rooney in the team and we'll play 433 this season.
I don't think he is capable of playing a deeper role or that he can develop a vision or passing associated with the very top central midfielders.
The last part is probably correct. We are massively overpaying because of a whole lot of other things than just his performances on the pitch.
His vision and passing are probably better than people seem to give him credit for. He's definitely a very good player, when pressed. Most opponents can hardly get the ball off him, similar to Vieira. He's obviously not on Vieira's level yet, especially in terms of leadership, before someone misinterprets what I am trying to say.
He's one of Juventus' best players and they are one of the best teams around Europe. His goals and assists have contributed a lot to their dominance domestically. What you define as world class might not be what he is, but I don't think there are many midfielders better than him.
I think it's decent but nowhere good enough to pay 100m for.
I have actually seen him lose the ball a lot when pressed. Though, I expect that to improve as he grows into the midfield role.
He is Juve's best player in a free kind of midfield role. Vidal and Marchisio did all the grunt work when he played with them.