A great story that has me thinking. I wonder if I would want to dim or turn off the brightness. It's a highly personal question/choice, as with anything in regards to the paranormal and spirituality. I do feel acting with respect and caution is a must when dealing with such.
On one of my many treks alone into the night when I was a kid, I finally mustered the guts to make my way into one of the long-abandoned houses up the road. (It is still standing, though the last time I saw it looked like a strong sneeze would knock it down.) There's a lot that went on that night but pertinent to my reply here is that I found a ouija board inside and ended up bringing it home.
I went online and found this image which does match the ratty box that the board was in. (But I remember the board itself being a different color from the one in this photo.)

I never did end up trying it though. I've always been a curious person but everything that I saw in the library said that it worked better with others (and my dog didn't count). My mother was furious when she found it and there were repercussions for my daring to bring it into her house. The board is gone now, it burned in a fire, but sometimes I wonder if, even though I didn't use it, maybe something came home with me. Positive or negative, that I can't be sure of either as there were times in that house that I was alone (physically) and felt safe and other times where I wasn't alone (physically) and was scared. The question is there along with the wonder.
Sounds like the beginning to a really scary movie! Yes, more than one person should have a finger on the planchette. I'd be pissed too, if I were your mom. Whoever used it could have left something behind.