Is this the first chapter/forum thread you've read? I did try and make this stuff clear, so... sorry if it wasn't enough, I guess?
But short answer: the mangaka worked in hostess bars, drinking places etc. when she was in college, and "Mii-chan" is inspired by an actual young woman she met who was also working at the same place, who had what the mangaka herself described as a "mild intellectual disability". (She was apparently certified/had a certificate of disability, and an upcoming chapter goes into what this means/what it says about the system back at that time.)
The mangaka has stated publicly that the majority of the story is fictional - like, in interviews with Japanese MSN under her own name, or on her own X/Twitter account - but that apparently the real "Mii-chan" did in fact do stuff like give out her real name/home address to anyone, even the sketchiest customers, and she did get in trouble with stalkers. And she could apparently barely read kanji, to the point she couldn't read the menus at the izakayas/drinking places in Kabukicho.
(As far as I know, the real "Mii-chan" was not murdered, and if she slept with any customers, I haven't seen the author saying that anywhere.)