I don't agree on that. The ideal line doesn't have a slow entry speed, only requires a late apex. That allows you to take a high speed into the corner, but requires you to stay on the outside a bit longer to ensure you don't compromise your exit. That leaves room that Schumacher used to pass Di Resta, even though Schumacher needed the help of di Resta for that. However, Grosjean wasn't even near enough to try that move and Webber covered the inside line enough. In my opinion he just slowed down too much, leaving Grosjean nowhere to go.
Regarding the overtake of Schumacher on di Resta, it was di Resta who left the ideal line in the first part of the corner because he didn't shut the door going into turn 1. Maybe he thought Schumacher was already on the inside, but di Resta steered in too late, compromising his corner speed and leaving room for Schumacher on the inside, who took the opportunity brilliantly.
It was the small error of di Resta that gave Schumacher the opportunity, not the layout of the corner.