I have a feeling this is going to be extremely unpopular, but...
What if SM is telling the truth? She could possibly have narcissistic tendencies and possibly has dissociative disorder. Mix that with the pressure she may put on herself for not being Gannon's "real mom". She might not have been a good mother - those issues would contribute to that.
Has anyone else here been absolutely terrified that you are going to be caught in a store shoplifting yet you've never stolen anything before? You don't even have any intention of stealing but you are just freaking out over it. That probably draws more attention to you and makes you look suspicious. I am very familiar with that and I've never stolen anything. Ever. I'm not sure how to word it but I think that could possibly apply if she is innocent. She's making herself look guilty.
Anyway, just what if she's telling the truth.
Edited to add reasoning: I really, really, really want Gannon to be a runaway. There might be a chance he's out there.