I like your fluff, it's well-though out.
About the Librarians.....
Maybe your Space Marines have developed an unfortunate mutation that only allows one-way warp tinkering; they can be affected by it, but they can't affect it themselves.
This makes them an anti-thesis to Chaos, and even more so the Daemon-spawning Word Bearers, and would even more fuel their hate, and could explain why the WB attacked them in the first place.
(These mortals have no capabilities to serve our Gods, and so must be eradicated!)
