Your comprehension is bad, MC was always human. It was the MC's dad that was the scammer and he killed him so he could ruin the girls. The lost of humanity being a thing is just a plot device because the author wanted it and has nothing to do with either world.
They couldn't tell they were different humans coming after them, so feared them all as one being and their fear of that immortal hunter made humans more powerful back then.
They skipped explaining how the whole clan is suffering the effect of inbreeding, the MC thinking he was dying when he got sick is because of a compromised immune system do to said inbreeding. No idea why they left that part out.
Jolly gee, I can't see any reason at all someone would want to keep their ex and the stalker that's blackmailing them into a relationship from meeting each other. And Kibe isn't a friend at this point.