For the Imperium, is this limited to Astropaths, or could an Imperial Sanctioned Psyker also receive it? They give other options, besides blindness, so I figure a powerful Psyker fighting alongside Guardsmen could benefit from the "security" of the Soul Binding to the Emperor.

As a second, do Chaos Marks grant this trait? You have, in essence, given your soul to the Ruinous Powers, and I could see you getting this advantage, in addition to the power granted, but don't know if the rules agree with me.

Well, the mechanics of soul binding and the background for the same term aren't necessarily the same. The Imperium reserves the term soul binding for the process of creating and binding astropaths. That is the only meaning of soul binding I have found in the rest of the 40k background.

The rpg rules apply the term more generally... and so what the rpg calls soul binding could be applied to chaos psykers bound to daemons or gods, alien psykers tied to some other power or perhaps even Imperial psykers trained in some rare and exceptional discipline.

Well, the mechanics of soul binding and the background for the same term aren't necessarily the same. The Imperium reserves the term soul binding for the process of creating and binding astropaths. That is the only meaning of soul binding I have found in the rest of the 40k background.

The rpg rules apply the term more generally... and so what the rpg calls soul binding could be applied to chaos psykers bound to daemons or gods, alien psykers tied to some other power or perhaps even Imperial psykers trained in some rare and exceptional discipline.

AFAIK, you can also be "Soul-Bound" to the Ruinous Powers (save Khorne); but in the context of the universe, Imperials probably doesn't even have a name for what the witches of chaos does to become "Soul-bound".

In GW's background, the reason for Soul-Binding seems to be to link the legion of Astropaths to each other so that they may transmit and receive telepathic messages through the Warp - something no ordinary psyker is capable of. It almost reads as if the Emperor basically acts as a sort of "hub", and anyone who has been bound to Him is simultaneously bound to every other Astropath who underwent the same ritual, thus creating a network. The 6E rulebook describes the ritual and its effects on page 165.

If you are looking to increase a psyker's resilience and security, perhaps check that other thread where I quoted some "failsafes" from the IG Codex entry on Battle Psykers.

Of course, all of this is just how it was described in GW fluff - it may or may not fit to what this RPG's books say. As always, I recommend to read and double-check the various sources and then decide for yourself!