(literally) a switch is flipped / (by extension) someone's mood or thinking changes, as if a switch is flipped in their mind

dark; dim / sombre; gloomy

change one's methods or ways of thinking

switch, a device to turn electric current on and off or direct its flow / (rail transport) switch of a railroad, point of a railroad / a change in method or thinking / (computing) network switch

入る: enter; join

配流: exile

room (of a house or hotel), chamber, apartment, flat / (sumo) a stable where rikishi live and train together under a shisho oyakata (stable master)

(と〜) thinking that […] (with the next clause explaining that the truth is contrary to expectations)

to seem, to appear

in my opinion; I think; I guess; upon thought; upon reflection; upon contemplation / conceivably; presumably