First off, the fact that we gain access to it by going flawless on the Trials of Osiris. Second, the entire plead is decked out much like Brother Vance's little area. Third, the New Monarchy team sent to explore it found items that show a clear connection to the Cult of Osiris

Anything else? I have a bit of an idea I'm not quite ready to share, and want to research everything first before I do

Based off of this info, it's pretty apparent that Osiris, or at least his Cult, inhabited the Lighthouse. Plus, some of the flavor text on Osiris gear ties him/the Cult to Mercury, e.g. "You can't pull an all-nighter when the sun never sets."; "The sun took my sight, thus the sun became my eyes."; & "In the mirrorlike crags of the Fields of Glass, a single mote of dust produces infinite reflections."

Damn, that does seem pretty conclusive. I was thinking, a couple things: Brother Vance was sent to rope us in, the Trials weren't the immediate reason for trying to entice Guardians, there was something else and the Trials were just a fallback; If Osiris was exiled from the Tower, and as the Speaker's apprentice as well as the Vanguard Commander (the position currently held by Zavala), why is the Lighthouse bearing the Speaker's banner? Why not the Osiris themed banners? What if the true intentions of the Cult of Osiris was for a select few Guardians to investigate the Lighthouse, to show us something about the Speaker?

Good question