When I was thinking about what might have happened to NG, I thought of Ted Bundy. Although he attacked younger females, some of his methods may be relevant to what happened at NG's house.
Bundy attacked women at home in a number of crimes. He acted alone. He carried them away himself in his own car, and disposed of them in remote locations. Some of his victims have never been found. Sexual assaults of various types were his part of his methodology. He also went back to the disposal sites.
At the beginning of his criminal career, when he was a young teen, he was thought to be responsible for kidnapping and murder of 8 year old Ann Marie Burr, in Tacoma, WA. Bundy lived in the same neighborhood. Ann Marie Burr was never found.
I speculate that NG might have been the victim of a psychopath like Bundy, acting alone, who lives in her neighborhood or is familiar with it for unknown reasons. There was an attempted assault in the neighborhood that could have been done by the same person. The suspect could be searching for victims in the neighborhood, and familiarized himself with female residents and their routines.
It may have nothing to do with SG, and be a situation where a neighborhood with low lighting levels, substantial cover from vegetation, and a victim unable to fight back were important factors.
In this situation, attempts to collect ransom would be by an opportunist, and unrelated to what happened to NG.
LE may know if there was more than one perpetrator from DNA, and if the ransom notes reflected unique factors known only to the perpetrator.
Just speculation, but I think it could be a lone perpetrator, and that the ransom demand was a separate crime by someone who wasn't involved in removing NG from her home.