Yes, but you guys have to realize something. This movie isn't gonna be all at once around march. Its gonna start being worked on very early, probably right now. Animated films take longer than you think.
And Oda would no way tell anyone something as import as "is so or so joining?" before actually confirming it.
If he did he would clearly not give a shit about his story.
Not necessarily - Toei has to ask Oda about whether or not they can "invent" a Devil Fruit if he plans on using it later on. Like, for example, while they were making the G8 arc they might've said "dude, let's give Jonathan a Giraffe Devil Fruit!" and Oda was all "No, I'm doing the Giraffe thing later".
Also, the "Free Will" ending with Robin had shots of her lonely and sad childhood (as did the "Bon Voyage" opening). Wouldn't that suggest Oda at least said she had an unhappy childhood? He didn't necessarily have to say "yeah, her entire island was obliterated by this thing called a Buster Call" but he could have easily said "Robin's childhood was very sad and she spent much of it alone".
EDIT: Then again, Movie 3 takes place in an impossible timeline. . .