I agree with being a bit wary of possible retcons. I understand why the Marines have been added: in addition to being popular with a lot of fans, putting them in the book gives the colonists slightly more of a fighting chance, increasing the tension (this is a 360-or-so page book, after all). My only worry is that it will drag River of Pain into the same mistakes that ruined Sea of Sorrows for me. Namely, the middle portion being dominated by generic, military type tertiary characters meeting their grisly ends.
I remain optimistic, though, because that newest summary says more about the Derelict than it does about Colonial Marines, and the exploration of the Derelict is what I'm most looking forward to in this book, assuming it happens.