Let me first start by saying that this is my opinion and I can't guarantee that you're gonna feel the same way once you play the game.
Ball physics have certainly been improved from last year, especially crossing and shooting felt much better. I didn't really like the ball physics in PES 2015 but I don't know why. PES 2016 just
feels more natural in this regard.
Shooting feels a lot more varied. I personally thought the shooting in PES 2015 was really bad most of the time, and they've certainly worked on it - it's a lot more satisfying to score, because there are plenty of ways to do it now!
The goalkeepers are much better so I don't think it's as easy to score as it was in 2015.
No, I only played 1v1 with a mate. I played with Brazil, France and Juve.
About the old school feeling - it's very subjective, I tried PES 6 recently and it's just so frustrating to play with 8 directions and so much limitations in the gameplay. As opposed to that, PES 2016 feels a lot more fluid. The one thing they've brought back is the feeling you get from certain situations - they just feel right and realistic. I scored a beautifully worked goal with Brazil which felt more satisfying than any goal I've ever scored on PES 2015.