Ignoring the vast majority of your post which so richly deserves ignoring, I'll focus on this idea of what decade it is.

You got it right that it is a question focused on the point of view of the police officer (and others in the system, if he detains her at all).

What you describe as what would happen today clearly goes too far, and I have said so. The system can use a lot of improvement.

The reason I have her mentioning the ongoing sex is to create a situation where the police have probable cause to think something really bad is going on. I have my doubts that they would just have let her go back in 1960 -- at least it's far from certain.

Current society has its limitations and drawbacks. It has the same drawback as an incumbent politician who has to answer for all the sticky issues that reality imposes, while the challenger can just paint the vision of how everything will be wonderful if only they are elected.

Radical child liberation is not a reality, so I raise this case as one where the wrong thing would happen IF it were a reality.

The vast majority of the time, the child, the parents and the state all agree on what should happen, so they're not interesting. "Yes, eating meals is a good idea." I've picked an example where the state (the police) is called upon to look at a conflict between (step)parent and child. I can make the case clearer by positing the current system as improved to the point where it has excellent foster care.

I see it as a case where among the alternatives a child perceives in a confusing sexual situation, she might very well make the wrong choice. There is a role for the state to hold onto her long enough to help her make a more informed choice.

the novel was NEVER meant to be read as a REAL narrative about REAL people.

True enough. Charlotte's death on the way to the mailbox struck me as the clearest case of that. But like any good novel, it paints some parts of reality with enough clarity to be of real interest.