I am betting on genetic/human modification and its failures. Something happened where a massive disaster occurred and in order to preserve humanity, scientists started to develop modified humans. Modifying creatures and human mixes as well, with less of an overall success at human longevity. However rarely, more of a random near if not immortal production would be made, thus the various sentient creatures like Genius. Witches are another form of human modification. They were modified to be mixed with technology (thus magic). These few superior humans were unstable though and because of their bodies surviving off of energy converted from electricity (assuming that remote charging is a thing) it would explain the immortality and the "magic". Likely they were combat models. Actually, I would not doubt that everyone is using some from of remote power as a psudo-life force. The "moon break" is simply their receivers either overloading from years of no maintenance or alternative power supply, or are no longer capable of keeping their genetic build up in tact. Without external genetic code, the bodies simply fall apart which causes their receivers overload and disintegrate their body.

Now after a while of genetic modification, humans adapted for a time, but society reverted to a hierarchy. Superior humans (witches) who were successes for the most part, saw all others as failures and toy's / food to be used for their own pleasure. The 2 flaws of the witches is that they are genetically unstable under a certain circumstance. This was a fail safe designed in case they rebelled or became too hostile. the other issue, is that they are incapable of reproduction.

As for the neck rings, those are how humanity showed someone was "stabilized". In order to be efficient, they genetically coded women to gain a neck ring if she has been "stabilized". One of the issues though is that only females would be produced as offspring. Genetic modification deleted males unintentionally. So in order to preserve humanity, the Kull were mass produced as an attempted solution. Basically mobile semen dispensing bots which were intended to be capable of finding unstable girls and stabilize them artificially (sex at this point was likely seen as a requirement of survival, not done anymore for intimacy/pleasure since nothing good would be produced). However a glitch in production occurred and made them overly hostile/aggressive and not capable of stabilizing females. This also explains their similar voice.

God of Deep Jealousy isn't a god or anything. It is a made up name. A nickname/"slang phrase" which got way too much traction. What they think is the God is actually a med lab. One that specializes in human modification (specifically in the physical appearance area). One of the advancements made was the easy changing of appearances. This machine could make you look like anyone. Thus was nicknamed "God of Deep Jealousy" as a joke on how Jealous people sometimes change their appearances to mimic others. This was also the machine which caused the genetic failures, and elimination of males. The machine changed people on a genetic level, not like how plastic surgery and graphing/transplants are used today. Female DNA may have been more stable to use, so they always used it. And the unintended result was genetic removal of the Y chromosome. Eventually men died out over the years. The "God of Jealousy" was decommissioned and stored away. MC's friend was stored with it as a sample of "unaltered genetics" so when in the future someone could fix it, they could still have good genes to use.

The only thing I cant explain is the time travel. My only thought is he was sent to fix things by even more future society. Yes he is not from he past, but the fixed society in the more distant future (kind of like terminator in a way), but a future where technology is only starting to reemerge as more and more stuff is rediscovered. This "stuff" is recovered tech which was stored in secretly so as to allow future humans a chance to recover with it if possible. It was then fixed secretly to not spread panic by reintroducing the dark history of society. So they do not tell him why he is there. They just need him to fuck anyone and everyone to restart the genetic stability so that society can eventually be revived. And fuck he does.