Spoiler for EP8 ????:
I think keeping the original name is the safest bet... I'm one of the people who heard the name only through spoilers in this forum, though. But Kotobuki Yukari already feels somehow familiar, and it doesn't seem to need as long explanation as Itouikukuro Reigonamu. It's just the kanji reversed and one of them turned to hiragana, right? Well if people don't know what kanji or hiragana are, it might get a bit longer, but anyway. I'd be fine with a Grimoire explanation, but I don't mind if you sneak in a couple of lines of your own.
I do like some of your suggestions like Yoshigune Maria for plot reasons (but I don't know if Ange would use that name out of respect or dare not to). Does Yoshigune mean anything? Yasugane Hiromi sounds fun as well. But does Ange know about Yasu's name? Would it just be a coincidence? "Cheap gold" might be fitting anyway (well those with no japanese knowledge might not spot the "gane"). Hiromi might be translated as something like "broad truth", right? Jean/Gean Otobe is a bit weird and indeed Ange Beato as the source sounds lame.
You don't have a real english version of "heso kande shinjaeba~" either. It's not really anything important. But I just thought up yesterday that something like "disappear forever" could work (foreva~)!
I don't know what to do with Chiesters. I've also thought that it's too easy to figure out if it's going to be romanized that way. And Siesta is what I got used to. But it seems to be the official romanization, and makes more sense that Siesta in the end, especially for those who don't know about japanese spelling.
I feel bad for spoiling so much of EP8 for myself already, but I'm sure there's still many reasons to enjoy the real thing at last. Can't wait for the weekend!
Witch Hunt, the best of luck with reaching the finishing line after all your hard work!