The article seems to say to be clear that there was no evidence of sexual intent (and I don't just mean they overlooked the obvious because there wasn't glaring proof they couldn't ignore, the article suggests they went to some effort to try and establish if that was the purpose).
So, reading between the lines of that Mirror article, I wonder if this is a case of Ava playing on normal adolescent insecurities and grooming the child into identifying as transgendered and convincing them to "run away" with Ava and Trevor. But the kid then realised it wasn't actually the escape from a horrible life that they thought it was it was a seedy trip in a car with two men who were not stable or at all aware of what a 12 year old actually needs.
Which would be in line with the safeguarding concerns people on here have had about the grooming role of transactivists who encourage questioning teens to talk to them not their parents and LGBT groups who mix adolescents with 20-something adults as though adolescents aren't vulnerable and nothing could possibly go wrong.