Shaymus, makes me start to wonder whether Ronald made a change of the locks after Misty's wild weekend. She takes off, he figures they're done, changes the locks to make sure she doesn't get in, and then they make up, Misty comes home, but she doesn't have a key to the new locks yet. Your thought about the front door needing a key is interesting and would explain why the back door would be used for egress/ingress when Ronald leaves. (ETA: Re-listened and Ron states that Misty was standing at the door, the door was opened. What we all assume is that she's standing in the front door, but I don't hear that; he just says he pulled into the yard and she's standing there. He does later state Misty told him the back door was open when she woke up, but again, I'm not clear as to which door she was standing in, at least from this video. "Yes, I never walked through the door. Um, I pulled in the yard and she ... the door was open and she was standing there waiting on me and I asked her what she was still doing up and she then told me that..um....the back...that she got up to use the restroom and the back door was wide open and that my daughter was missing.")
So if Misty, fresh with new locks, wants to make sure the back door doesn't close and lock on her if she goes outside maybe for a cigarette and to talk on the cell phone with friends or whatever, catching the door on clothes, bricking the screen door so it doesn't lock too (she could imagine it would if Ronald has gone into the same sort of litany about the doors as we get here; she'd wonder whether they would all lock on her if she went outside).
Curious.