Yea, I dont really understand why you'd think shes supposed to be all CG. Shes supposed to look pretty much human except for a few small things. I could easily see PARTS of her being CG, but I can't imagine that anyone could render someone well enough that she'd be able to fit in with other real actors.
Why on earth would they need to make Alita CG? I mean look at her - Some decent prosthetic overlays for the arms and you're done. As for the ideal actress for Alita... I'll have to get back to you.
I have a book with the 'Alita' over the very top of the wings, in case you wanted a better/HQ scan As for doing her CG, if it's true, they might be doing it that way for the animatability... though doing it well is something else entirely