An interesting point was made at the hearing by an MI6 agent. He stated she was 15 at the time she went abroad, she was intelligent with anticipated grades in exams of A and A+. She was not uneducated, she knew or must have known what IS were doing and the horrors they had alrwady committed. He stated she made the decision to go.Exactly. In an ideal world I'd want to see her prosecuted for treason and terror offences and throw away the key but the reality is because of the reasons you mention there's a massive risk of her not ending up in jail should we let her back into the country.
The next best solution is to do exactly what we've done. One less terrorist on British soil and some people are acting like that's a bad thing.