Both stilgar and chani have been cast and they very small parts in the first half on the story. If they announce the actors playing shaddam Iv or irulan in the next few weeks, it will make it likely they will be filming both parts simultaneously
Although the basic plot is not particularly hard to follow, a the inner workings of the whole universe only become more apparent with subsequent readings. But once they get to Arrakis and the sense of the place begins to be conveyed, the book truly becomes wonderful. And once they meet the Fremen, it can get mesmerizing
No, that's Kynes. They meet Stilgar when he spits on the floor in the Arrakeen Palace, after one of his men was killed and Duncan had already spent some time with the Fremen
Actually, according to the book, Duke Leto was 51 during the events depicted, and Lady Jessica was 37.
Viggo is now 60 and Rebecca is 35, so I don't think it's that much of a stretch