I believe she gained the awoken's trust and the queens trust under the order of her true queen. The awoken "waiver between light and dark" which is why I believe they did not sense anything wrong with eris. Having the awoken's trust then gave her credibility with the vanguard so they overlooked the darkness inside of her. I believe that one of Oryx's sisters put a worm in her, and shielded her from crota. This is really the only thing that I have come up with that explains how she could survive down there for so long. PS. Thank you for responding, I'm a giant lore junky and I love what you and your group does.