I've been reading a lot here, decided to chime in. Weirdly, like SG, I have a journalism degree and am a lawyer (though not criminal).
Based on all the info I've seen so far, including AB'S allegations from her tip, statistics of crimes being a known person or family etc., I am thinking it is SIL in my opinion. Knowing where cameras are and to remove them, seeming to know about a back door, being in the house for about 40 minutes, makes me think it was someone she knew. They didn't rob her during that time, maybe even were speaking to her during that time, and she didn't call 911 during those 40 minutes.
Also motive. I'm thinking maybe there was resentment with caretaking responsibilities because the siblings live far away (and one is off living a glamorous life). Life insurance could have been a big motive too (also explains why the home wasn't robbed, they'd inherit belongings anyway). I hate to say it but AG would be the only alibi for SIL being gone in the middle of the night. Maybe she was in on it? They were the last ones to see her, so they say.
I also think if it was a real kidnapping for ransom originally, the ransom note would have come much sooner, as soon as the story became public. Maybe once they realized how big this story became in the media they decided after the fact to throw that ransom note in as a distraction or just a stupid, scared move. I have middle schoolers and they know more about bitcoin and computers than me just through watching YouTube. I'm sure a 6th grade teacher and parent of a teen/pre-teen would pick up knowledge like that, and science teachers these days need to be pretty tech-savvy. Or maybe the ransom notes are hoaxes that are not from the perp. Again, all just my opinion.
My other thought would be a worker very familiar with the house, who discovered who she is/who her daughter is and who decided to kidnap her to get money out of SG.