I'll try to answer the questions as quickly as you asked them.
I'm not a deep expert in the public participation part of EA. I won't say that much on it. I would say this though. The early planning stage is vital. By providing that clarity, scoping, and direction for an EA, you actually solve a lot of problems later. It is a measure twice, cut once kind of approach.
Doing the early planning is really important. Again, the proof will be in the pudding. If the agency does it right, it should scope the project in a way that all the key concerns identified by experts and affected communities are identified. At the end of the day, you should have an outcome that gets more social buy-in and is a better project. You can't legislate good performance in an act. I think the goal is good.
As for public participation, most of the cases that I have seen that have gone to court have been a result of a group or a large community feeling that their legitimate concerns about a project didn't have a venue to be heard. Most times when people feel that they've had a chance to air their concerns, that they were listened to impartially, and even if they don't win but were at least taken seriously, they can accept the outcome.
I think it is vital in terms of outcomes.