You guys are one the side of the Great Rift that is cut off from the Astronomicon. It's kind of hard to call upon those other chapters when the psykers aren't able to send out long range communications, and any ships that try to make long range jumps end up lost in the Warp.
I mean, it's not like you can't plug any holes at all, you just have to do it one at a time. Beyond that, I'm not really sure what to tell you other than "Welcome to Warhammer 40k". I thought that the fact that the quest is set in a period where the Imperium is currently in pretty much it's darkest point since
the Horus Heresy made things reasonably clear. There's a
reason why the introduction to this quest ended with a link to a video that sums up the entire franchise as
"Life sucks, there's only war, and you're probably going to be eaten by Tyranids." after all.