So who are these "Ten Simpletons"? I can't help but think they are victims of Shae's attempts to create a circuit that keeps his own soul from Damnation. How else would they come to know with such accuracy the nature of damnation?
Or did they somehow collectively possess the Judging Eye? But then why does the work end up also being called the Inverse Fire?
Yeah, unfortunately there is no other mention of them in the series. But that is probably because there really isn't "anything" for to them. They are probably a narrative devise to expound something of a Foucauldian view on notionally "madness." That is, what is termed "madness," normatively, might actually be a form of insight. Labeled "simpletons" allow normative society to marginalize them and the implications of what they "see."
In any case, maybe one (or more) of them did possess the Judging Eye or something like it. On why it would be called The Inverse Fire, I think you could imagine that somewhere along the line of years, the term could have gotten brought up, from various "occult" or "cultic" sources, but devoid (or simply divorced from) it's exact original context. So, someone stumbles across the term in fragmented Nonman sources and says, wow that seems to apply to that ceiling, never realizing the depth of that connection though.