^Some good choices, though I think Stephen Lang would be a much better fit for Gurney than Stilgar.
I was thinking again about the Baron, trying to come up with a slightly younger Brian Cox type and suddenly John Carroll Lynch leapt to mind.
For Chani I'd go with Hailee Steinfeld as right now, she just about the right age where she can play younger so when the two year gap hits in the middle of the book and she's suddenly a mother, it'll be a credible transition.
The real challenge is casting Alia. Probably the best approach is to find a talented 9-11 year old who's small for her age. Maybe even use some forced perspective trickery to make her look even shorter, or find a young small-person with the right look and acting talent.
I too considered Hailee Steinfeld for Chani and while a good actress, she doesn't look right for the part.
I didn't even consider an actress for Alia, think a combination of a very young actress and some CGI could accomplish the role.
I've written several Dune screenplays over the years. Every few years I take a stab at it, seeing what new insights I can gleam from the book. In fact I'm starting a new version in a few weeks, this one is based on an old idea I had about producing my own Dune series, I'm going to explore that again.