I've said it before and I'll say it again, the case this one reminds me most of is that of Laura Houghteling. And that was a homicide. I think NG was killed in her home, and the body removed for some reason by the killer, probably to remove evidence on her body and buy time.
I don't know about motivation, but it doesn't appear to be theft, at least not of items from the home. It could have a sexual component; Bundy did similar home invasions where he took the victim with him.
I don't think any of the ransom demands are from the kidnapper; I think they are the work of opportunists. A genuine ransom demand would have come in immediately, not hours or days later.
MOO