The thing I'm wondering is the timeline. When is this story happening in relation to Helck's story? Because in this story the world seems really shitty while in Helck things seemed really good when the story ended.
The closest thing to a hint at a timeline is based on some of the content in Helck and in the Piwi miniseries. When we first see Kohaku in her little one-panel cameo near the end of volume 7, she looks quite a bit different, most notably wearing a shawl that conceals the top of her head. At the end of Piwi, we see her briefly in the same outfit (and basically even the same "walking away from the PoV" circumstance), then she removes it and we see the face that we now recognize as Kohaku for the first time, as she stares looks out over the horizon.
If we assume that this transition of outfit is meant to signal how she was in Vermilio's memories vs. how she is now, and given that the Piwi miniseries happens as a side story around halfway through Helck, then it's probably also a safe assumption that she took on that appearance just before chapter 0. Which would mean that the present-day Volundio series is taking place a little more than a century after the end of Helck.
We also can infer from comments Kohaku makes during the fight with Lunathea that the current locale is pretty distant from the Empire (and the journey began on roughly the opposite end from where Helck takes place anyway). I'd say the distance both in terms of time and place, probably accounts for any major shifts, without really negating Helck's own happy ending.
