Jay, here are some suggestions I have for shooters outside of Xonotic. Some have been mentioned/suggested before (several are on your list), but I can verify these are all still maintained, and I could get them installed on Linux as well as FreeBSD, so pretty much anyone should be able to play. I'm working on a list of some others that are not shooters. These are all at least relatively modern, I'm not 100% on the online multiplayer, though. A few are derivatives of others, and Open Astro Menace is a space shooter, not an FPS.
Black Shades Elite
Open Astro Menace
EDIT: There is also a game fairly similar to Star Conflict I'm playing with, called Vega Strike. Only the SVN builds have multiplayer right now, though, I'm not sure how stable it is.