Do you have a source for this? It's interesting because in 1996 there would not be much on the internet, which was still in its infancy then. Ruth would surely have been more likely to look at paper records (she went to London to find her mother's death certificate) or microfiches of old newspaper articles, if any were indeed written about her mother's death.
I agree with you that this was a planned disappearance -- everything points to that -- and that Ruth likely had help. Her case has never been reclassified as a murder despite her being missing for decades, which suggests the police also believe this. I think if Ruth had informed the police or Missing People (where her profile is still up) that she was alive and well, they would not have an open misper case for her. And they would inform her family, even if Ruth didn't want them to know where she was.
The police have said that there were credible sightings of Ruth in the Dorking area after her disappearance and her parents' belief that the person in the CCTV was Ruth would hold some weight. So, it does seem the police believe that Ruth is most likely alive or was after her disappearance (it was a long time ago and anything could have happened).
I do believe Ruth had help (whether from her friends who have stated publicly they don't know what happened to her, or from someone outside of that group of school friends). Whether the person or people who helped her know what happened to her or where she is now, who knows. Young people who run away are vulnerable. I would strongly recommend reading Shelley MacKenney's book -- Shelley ran away when she was older than Ruth and considerably more streetwise, and narrowly escaped being abducted:
Her book is on Kindle and is really worth reading to understand how hard being a missing person is -- Shelley's family never reported her missing to the police so unlike Ruth she did not have that pressure. She could use her NI number to work, which Ruth could not.