Here it goes:
the device theory is kinda nice. But there is one question
From where the f**k william got the device in fazbears frights?
Maybe, he got the device when he was springlocked, and the children he killed never EXISTED (except for charlie)? We have NO cutscenes showing him kill other kids. Only charlie. Maybe charlie can switch animatronics. And the proof is that in all fnafs only one animatronic makes a step and then another. NONE do it in the same moment. We can see only ONE ghost moving in the springlock cutscene and that mightbe charlie and the other ghosts are fake.
Also if you ask about ucn that "cassidy" "made" it makes no sense to me. From where cassidy would magically appear in FFPS? Charlie was right there, and could teleport william into ucn.
MAYBE THE ONLY KID THAT EXISTED WAS CHARLIE? THE ONLY ONE THAT HAS A DEATH CUTSCENE?