Ah, your video helped me to find what I had been missing! And with that final clue, I managed to complete the game (I think, at least)!
(To be specific, I had managed to forget about the third room holding a little controllable figure, and somehow had not stumbled upon it again after discovering the related mechanic.)
So, thank you for that! ^_^
In case you want to find out how to proceed--or anyone else does--I'll add some hints into a reply to this message, I intend...