Everything is used very effectively in this. I'm not sure if you need the activation button since you really only use it for the key tile trigger - which could just as easily be based on the player position (like the door is). If you keep it, I would say use it with the door too. It felt strange that I had to trigger the key, but not the door.
I played it about 5-6 times but really only made it to level 3 each time with limited light. I really enjoyed the level design and I think it does a great job of teaching while you play. I would just consider lengthening the overall light time or adding a few charging points. Right now I feel like it can be luck whether you run into a charge, a key, a door, or the stalkers.
Speaking of the stalkers: they terrified me. The music, the animation, the effects when they appear. All great stuff. This game's atmosphere is awesome. The moment when I first saw one I could feel my pulse race and then ease off when I made it through the door. That said, by level three the one that's there doesn't seem to leave you alone. Can you outrun them? Or hide? I didn't seem to be able to, but maybe I was just doing it wrong.
Anyway, seriously great job. Congratulations on submitting, thanks for playing mine, and good luck.