I didnt catch up with all of the translated novel chapters, but here is, what I know by now:
The administrator D is Wakaba Hiiro, a female class mate and was the one, who reincarnated her classmates. She is also a "god being" and until now I dont really know why she actually went to school. It seems she wasnt affected by the explosion, but this wasnt explained until now.
Kumo ACTUALLY REALLY was a spider, which happened to be in the classroom and inherited part of Wakaba's memories, because Wakaba transfered a part of her soul into Kumo when she reincarnated. Later on she becomes an arachne with partly Wakaba's appearance and later even becomes something like a god, like D, only much "weaker". Also I dont get, why Kumo has these socialization problems, because it totally contradicts "D"'s personality. Maybe because of her former spider mind? It wasnt explained until now.
The demon king (Ariel) isnt Kumo, but basically the first spider ever in this new world, who is currently the strongest being alive, which isnt a god. Before Kumo's transformation into a god, Kumo and Ariel had a fight to the death, where Kumo's "Parallel wills" merged with Ariel, so Ariel became something like a female version of "Gotenks" from Dragon Ball Z.