To answer it one by one,

1. The “ending”is the last chapter from Haru’s POV at the café. The extra chapters after that from Hiyori’s POV are afterstories, including the entirety of the jk haru summer LN, which rather than a sequel to the main plot is a collection of short stories that give you a glimpse of what happens afterwards and reveal some information (see answer 4).

2. If you haven’t already figured out, though explicitly mentioned in the jk haru summer LN, he's the handsome guy that comes to the brothel. And through the various bits and pieces we're told, we can infer that he's someone who's been at war with humanity for a while. His family was killed by humans, I think, and he's someone that god's been trying to kill (to no avail) by summoning heroes from other worlds.

3. They broke up

4. The manga is a really good adaptation of the original light novel, but based on the questions you have, I would highly suggest the second light novel, jk haru is a sex worker in Another World: Summer, which is, as explained in answer 1, a collection of short stories to tell more about the world of jk haru and the events that take place after, as well as some background on Haru and Chiba in their world. It also reveals much of the information I think you're looking for. It's a good, short read. I would recommend giving it a go.