I always assumed the priests were human, especially since they make an effort to gather eyes for their lanterns. Checking wikis, I can't actually find much lore about them, but unlike monsters, they identify you, warn you, then attack you. So they're probably sentient and the scythe guy at the UCW is guarding the door.
I am assuming that Choir is meeting their goal that night by the fact that they're all mostly gone. I think their goal was to get someone pregnant with a God. This goes into speculation more than anything, but I think the Choir and Mensis were enacting their plans the same night and you're just sort of caught in the middle. Mensis used Mensis cages and actually visiting Nightmares to speak to Old Gods - as in, they talked to Gods directly. Look at the description for the Rune Tool.
So Bergenwyrth tries talking to Old Gods, then Mensis is like "Hey, what if we tried to do this with sacrifices?" and burned themselves to do it with mensis cages. The Choir seemingly involved the Celestial Emissary somehow to speak with the Old Gods.
Again, speculation, but doesn't the name "Celestial Emissary" strike you as odd? He drops Kin Coldblood, so he's clearly not a great one, which puts him at level with Rom. He's also, well, weak as hell. Rom was a tough fight, but not because of Rom itself, but because of the mobs. Celestial Emissary just kind of vaguely swatted at you.
If we assume Rom was a human made into a God, and Micolash kind of implies that to be the case, I wonder if we've been looking at Celestial Emissary all wrong. We're assuming it was an Emissary from the stars, rather than one for the stars. It's strongly implied (about as strongly as this game implies anything) Iosefka can make Celestial mobs from people. What if the way they chose to communicate with the Old Ones was by creating their own?
This is all a really rambling and roundabout way for me to say that I kind of wonder if the Choir was that mob you fought in that boss fight. Obviously, totally speculation, but it's why I think their goal ended that night. The plan was to go to the Old Ones and birth another one, I think.