Just some tidbits of my observations and opinions. I hope it helps.
It is very likely that the Jaime Fraley case and the Jennifer Rivkin case are not related. I say this because of the circumstances and the areas of which these two girls were last seen. At the time of Fraley's disappearance, I lived in the Brentwood Chase apartments on Lowell Bethesda Rd. which is about 2 blocks from where Fraley lived and was last seen. The apartments that she lived in was not your normal apartment complex. It was only one building, very small and very run down. The area was riddled with drugs and ex cons living there. I believe that it was stated that she was involved with an older man who was a felon that was later found dead in the trunk of a mutual female friend of Fraley's car. Apparently, this man purposely got into the trunk to follow this woman, got trapped in the trunk, and died of heat exhaustion. The whole story was pretty bizarre. I think this man was just a friend of Fraley's. Her boyfriend was locked up in prison at the time of her disappearance. Anyway, I believe that she may have gotten mixed up with this older friend of hers and something happened. If not that, she may have fell victim to her surroundings and was abducted.
With Rivkin, her car was found along a pretty dangerous area nowhere near where Fraley was last seen. Dixie village is home to a couple of sketchy bars and drugs also riddle Franklin BLVD. with an infinite number of thugs as well. Its quite possible that she got randomly abducted by some lowlife.
My overall point is that Gastonia, like most other cities, has some bad areas where women can easily turn up missing. Traces of these women were both in and around such bad areas, although nowhere near each other. The only connection the cases have is drugs, Rivkin being a known addict and Fraley living amongst a drug haven. Other than that, and given the fact of the high volume of criminals and drug addicts in the city and the nature of the areas, it's really hard to pin these cases on one particular individual. The same applies to the other cases as well.
Of course I could be completely wrong. Being a resident of Gaston county my whole life, pointing out the high abundance the area has of scum is stating the obvious if you are familiar with the area. It could have been one in the same that committed these crimes, but I would bet against it.
Take it for what it's worth.