For anyone wondering, the author's intention when using these european names is to get a european feel to them, so when translating, they should be written as they would in any european country where they might originate from (Walford comes from english speaking countries, so it should be used like that instead of warford, even if the katakana is ambiguous).
Regarding the "lack of ruthlessness" in the jp novels compared to CH or KR ones, it's simply because the target demographic is different, it's like comparing Detective Conan with CSI, one's target demographic is aimed at early teens up to young adults and maybe beyond that, while CSI is aimed at young adults and beyond, though there are other factors too, not just age, but gender, social status, etc. Chinese and Korean novels seem more dark because the ones that get uploaded are those for a different demographic than Japan's ones like Kenja no Mago, it might be even possible that there are no publishers editing for those targets too.
As for what these demons are planning, Shin's powers are far too great, and they don't yet understand what they are, in fact, Shin's ability to imbue extremely complex effects onto items is on the level of a god, so he could very well dispell demonization, or even dispell the adverse effects of demonization, such as skin and eye color, as well as the madness enhancement, he's just having fun, though with what has happened now, maybe he'll react in some interesting way. And I don't think they want to demonize him, they likely just want to mess with him and somehow get him out of the picture so they can do some stuff to "mankind" (they're just buffed humans, I still don't get the whole "who will survive, us demons or the humans", it makes no sense...). Demonization, as I said, is unlikely, Walford has too much magic power, so if he hasn't demonized yet it's because he either can't, or he already is but without the adverse effects.