Just to add to the "point and click players found mirror accidentally", there's a bit more... To enter the mirror you need to be looking through the fireglass and jumbling, right? I think most players would check most stuff with the fireglass as you know it shows new stuff. And, if you go from the Omen room, you will be jumbling to get through the metal gate. In my experience, as you needed to jumble multiple times, I ended up always being jumbling. So meeting both conditions is fairly easy. In fact, the first time I played I tried to interact with the witch in the mirror and failed, because I was always jumbled. Next time I played I discovered that you can interact (sort of, doesn't say much) with the witch by not-jumbling. That's how it got discovered accidentally.
Great game and good luck with the jam!