At the very least, I don't think Riri ever killed Ooma. There's obviously a lot to discover, but if that sequence really does mean this is the fourth time(line) they've met, it would be plenty traumatic enough to lose the person you love three times, justifying her expression on the last page.
If I want to get really out there with my theories, given the sequence of the panels, maybe the Demon Lord incarnation came first and she kept finding Ooma in the mundane world after that. From there, the possibilities are A. Their souls (or maybe Riri's physical body itself) crossed over to the mundane world, possibly through Riri's own actions, or B. This is in fact the same world, and all the more "fantasy" elements have been forgotten or even intentionally erased.
...It's probably not B.
There's also other details to be speculated about. What if Ooma remembers the Demon Lord incarnation every time but not the subsequent incarnations? Is this the first time she's met Zai since everything that happened before, or has he also reincarnated previously and is intentionally hiding it? It doesn't seem like it but he could just be a good actor.
All that being said, all the speculation comes from a lack of context given at this point in the story. Literally everything I've commented on in this post could be completely misguided.
I just hope we get to see the full story the author envisions... And that it has a happy ending!