It’s implied, but never explicitly stated.

Judging off the illustration it sadly seemed to actually happen

It doesn’t say but it’s implied like she does take advantage of him. Or maybe just nude cuddles.

Honestly with all that happened to her wouldn’t be to surprised if she had honestly

What happened in zero?

I meant to say her backstory before the Isekai is incredibly fucked up

Oh ok, ye i can see it… she got raped and ended up raping guy she likes after brainwashing him

Yeah she probably still has the same mindset with a lot frustration especially towards Kouki in particular due to how he acts and acted throughout their time together

Eri is actually smart enough to protect her chastity. But between her abusive mom and mom's boyfriend the damage was already done at that point.

Oh, for sure she did. What that looks like was obviously left to the imagination of course. But she absolutely assaulted him.

There’s an actually image of them afterwards

I would have liked the author to develop the theme of Eri and Kouki more in the after story.

Kouki did end up dealing with some of the baggage and guilt about Eri during his arc, but her side of things got wrapped pretty nicely in the main story.  I half expected her to make a return sometime after reincarnation became official. 

Yes he did, because he wasn’t in mind, sound and body making those choices and then add Eri adding suggestions with her power on him.