Been a while since I dropped by -- browser messed up and I got busy and didn't fix it. Lazy in my old age.
I can't really speak to some of the ins and outs of the uber-plotting in the A, the V, or the AvP series. If you don't want to do the research on an existing world, it's much easier to make a new one up.
My most recent foray into was Turnabout, and that one an original idea -- using "original" loosely. I offered up four or five storylines for an empty slot they had, and IIRC, we wound up combining two of them into the book. My daughter was going to write it, but got into a time-crunch with her Aliens book, so I wound up writing it instead.
Movie universes tend to hold the movies up as canon and the EU stuff in comics or books or whatnot as less so, but sometimes, they like an idea and borrow it. Some of the material in a book might find its way into a movie, and thus becomes more established.
As far as I know, nobody sat down and worked out a backstory on any of the movies past what was necessary to get the pictures made originally. Predator II stuck that xenomorph skull into the mix, but I don't know if that was before or after Chris Warner and Mike and Randy came up with the AvP concept.
The Wilks/Billie/Newt/Hicks stuff had to do with legal rights and that it was later spoken to was probably as confusing as it was enlightening. Was what it was, and what we had to work with.
The book market is soft right now, and I haven't heard about any new ones coming out in the A, V, or AvP universe. If the next movie gets made and does okay, I expect there will be more of a demand for new novels and graphic novels.