Alright, I reread it. My thoughts along your points:
1.) My best guess is that it's Sibawûl, or some other member of the Scalded. This comes from the line, "a shambling line, nude, white where not soiled or scabbed or welted." It would also make sense why whoever it was has no teeth. Although, Akka has the same dream in WLW, Chapter 3, where he says that his teeth were "beaten from his skull." That doesn't prelude it being due, in part, to the Scalding though.
2.) The fact that whoever it is is referred to as a "stranger" to Akka kind of implies to me that it is not the FA, but the Second, which is why he wakes up and says to himself, "The Second Apocalypse had begun."
3.) I'm not sure, but the sreams inply torture. The abrupt cutting off of them, death. So, they are indeed killing them, to stoke the Inverse Fire?
4.) Which of course, brings us right to it. The Golden Room. I do believe what we are seeing is the Inverse Fire. Why is it Inverse? Well, Akka says in the dream: "to flee degradation on had to flee the World entire, to become a flame that burned no fuel." Also, the Golden Room has a "ceiling of flame." "A single glance exhausted his daring, a peek into fires burning upon fires, a bottomless regression." So, a fire that is above, a fire that burns no fuel, a fire that is an endless regression. It is inverted from how we would think of fire and burning and Hell itself. Hell, below not above; also burning as consumptive and transformative is how we would experience or describe fire in the real world.
Further evidence that it is indeed the Inverse Fire the Nonmen are staring at: "Tears enamelled their cheeks with furnace reflections, silken wings of saffron and crimson, damnation signed in passerine lights. They paid no heed to the mortals chained in their midst, for they were every bit as enslaved."
I have no idea what "passerine" is supposed to mean here, the only definition I could find online refers to birds. However, it sure does seem like it is the Inverse Fire they stare at, enslaving themselves.