So...I guess the most likely scenario here is that weapon that Himeko used had some sort of poison to temporarily put Chikane in a near death state right? I vaguely remember seeing someone mention years ago on a form, a point in the webnovel version (which I never actually read) where Chikane is trying to get back to the Island to stop a marriage ceremony so I'm guessing that we are just before that part and It'll probably be revealed soon that It's more than just "Himeko was secretly evil" because that's always how it goes with these 2.