Jackson can't use The Silmarillion as The Tolkien Estate owns the rights, and considering they spent years getting royalties from the movie studio who claimed none of the LOTR trilogy movies made any profit, it seems he will never get his hands on them. I think their hold on the rights runs out in 2043, so I've read anyway. You've assumed right though that content from the appendices will be used to bridge the gap.
fixed it for you..
it's all about the money, money, money
-- JRR Tolkien sold the movie rights for Hobbit + LOTR for royalties off profits
-- Movie studio claimed the movies didn't make any profit (accounting tricks)
-- Tolkien's son and head of the estate took them to court for years before he got a nice settlement
Tolkien's family are all feeding off his literary success, as you would expect (nothing wrong with it). So it's all about the money, not the "artistic" differences.
Tolkien's son, who is in his 80s, holds the helm of all rights for the remaining books and thousands of pages of his father's work. He produced Turin and the Silmarillion out of it all these past 40 years. I imagine when he passes away all the grandchildren will sell off the remaining works and we'll get more movies and much more content that we've never seen before.