
It was said somewhere that Kurokawa is 18, and that Shiba-chan is 22... and then the manga said they are 7 years apart... ????????????????
Plus what was that about the apartment? Kurokawa had a grudge against his brother about the apartment. Did I miss it somehow?

It's not really that you missed it, but I think were not given heaps of info, so it's kind of guessing from the hints we have.
I think the age gap he's referring to is with his brother.
Remember in like the first or second chapter he's on the phone and he's like "I'm not coming home blah blah its your fault". Pretty sure his brother got him an apartment and kurokawa was refusing to see him.
But afterwards they met up when his brother ran in to him and made up and I presume he moved back in with his bro (hence why he said she shouldn't of cancelled the apartment).

Is the term 'man-eater' a figure of speech, or are they really monsters?

They're most likely Incubi. A type of demon that uses sex as a means to feed off human life force, usually until they kill the person and move on to the next one. But for this manga, that doesn't seem to be the case. They're capable of loving and bonding with ONE specific human. Kind of like omegaverse lol
Absolutely need more!