Ok, so about Shachiku returning home at the end of the story:
I do think he will, given he's so excited about speedrunning, and soul-crushing work aside seems to have been fairly happy in his world, unlike most isekaiees. So what if, instead of him staying with the girls, the girls go with him to his world?
Ada has secretly been longing for freedom and I think absolutely enjoying her time with Shachiku (and I think people claiming that she's out of the running are speaking prematurely, especially since there's still development ahead), and even Kiira feels free from the pressure she's used to when with him, so it'd be really sweet if, once everything is settled and they aren't needed anymore, they got to travel to another world and experience the joys of speedrunning themselves.
Also I just really don't want them separated, okay? No I'm not coping, you're coping.