This is really messy and it gets messier at every turn. I feel SO bad for the family and NG.
I have my own suspicions about who's involved (and I do believe multiple people are involved), but the longer this goes on the less sense any theory makes.
SIL - How and where would he be able to hold NG for this long, while also avoiding the suspicions of a swarm of family, media, and law enforcement? Even if he's involved in some way, he'd have to have gotten in with the wrongest of wrong crowds to know people who could pull this off. Maybe if NG isn't alive anymore some of these questions go away, but then where is the body?
Workers/Acquaintances/Fans - The further this gets from being a robbery gone wrong, the less likely this angle seems. Whoever did this seems to have detailed knowledge of the house, NG's routine, and cybersecurity. They're seemingly evading even the FBI with no end in sight.
Foreign Government - I thought this theory was kind of crazy, but it would explain why the kidnapper is so adept at high-level almost militaristic operations. What's the motive, though? I just don't see one.
Crime of Opportunity - This is seeming more likely to me as the week drags on. Someone like BK in the Idaho murders who wants the opportunity to prove that they're smarter than police and they can get away with things. Almost like a test of their skills. If this is the case, I'd think they lived close by and probably became aware of NG and her relation to SG incidentally. Maybe they figured that they'd be able to do the kidnapping and, as a bonus, maybe extort some crypto and attract national attention through the SG connection.
I really don't know what to think anymore. I hope the FBI knows WAY more than they're letting on, because none of this really makes sense if they don't.
MOO