Happy to see this solved! I've been following this since 2014, it will be strange to no longer sleuth it.
As I suspected, Kimberly is not 1) an illegitimate child of Terry Turner 2) a Howder 3) a cult escapee 4) a missing person who was reported. She was, as I would have bet money on, a person who simply wanted to "start over".
In his defense (yes, it's a he) S/S and I corresponded privately last year, and the additional details he gave correspond with Kimberly's story. He said she had been living with her family and left to live with relatives when she had issues with her stepfather. Then she left those relatives to live in California, then Texas. The name she gave S/S in 1988 was made-up, Velling confirmed this after S/S called in the tip. The woman she was living with wasn't her mother - that was another lie she told.