Here me out: We all know that at the end of Monsters Inc Sully saw Boo at least one for time. And after Boo relised Sully wasn't coming back, she became obsessed with finding Sully on her own. Then she rembered the doors.
Clearly, the kids the monsters scare weren't in the same time or universe. So Boo found a way to harness the power of time travel through wood! This is why she widdles through out Brave. She is experimenting different things to do with wood.
By the way this is also why the banishment doors in Monsters Inc are mental. It harnesses the magic of time travel for a short period of time then dies out. Why is she in the dark ages? The will of the whisps hold the key to the time travel. The key clue that she is the whitch is that there is a carving of Sully you can briefly see in Brave.
This also shows why she vanishes when she walks through a door. She goes to a different time period. Does she ever find Sully? Well I like to think so.