norinew, I believe your instincts with mudgirl are good.
I am also afraid that any hopes that she’ll be able to treat this as just a bad incident that she handled well, without getting her any extensive counseling is likely to be in vain.
And the thing that pisses me off, and makes me mad at the world, is that my conclusion is not based on anything she might be feeling now. Nor because you could have possibly handled the situation any better. It’s because, willyenilye this is going to become a huge event in her mind, simply because of all the unavoidable attention that will be focused on it, and her memory of it, before it’s allowed to rest in the past. Again, I want to emphasize this prediction is not meant to be any kind of criticism of you, your actions, or even how the officials will be handling this.
Just look at the events from mudgirl’s point of view - when this happened she was taken to the police station, for a long time, and even had to talk to a policeman (or woman) alone. Describing what happened, and being asked to be as complete and precise as she can be.
And this is going to be repeated time and time again. First the CPS investigator. Then I imagine the C3I will be next with their own investigator. The DA’s office may or may not need their own interview with mudgirl. I’d expect at least one defense appointed psychological exam of mudgirl’s state of mind, as well. Especially focusing on her state of mind at the time of the incident.
I believe that the unavoidable effect of all this, will be that mudgirl is going to end up convinced that what happened to her is a big deal. And from there, it’s going to be a short mental trip to thinking she did something to deserve it, or to thinking that she’s damaged from it, or any of several other equally damaging modes of thinking or self-assessment.
And this assessment is without factoring the possibility that some of her mutual friends/acquaintances with the boys may blame her for what happen. Or that the families of the boys try to organize smear campaigns. 
In the long run, I’m afraid that I expect that mudgirl will need counseling. I don’t think you should be forcing her to talk about things more than you already have, except to let her know she’s not in trouble and that she will have to talk about things again with some other people. Emphasize that you love her, believe her, trust her, and that she’s not in trouble.
But expect there will come times when she doubts all of those things. 
My advice remains to keep things as low key as possible, while you wait for the CPS investigator to visit and you have a chance to ask for more educated guidance. But I think it is unreasonable to expect that you’ll be able to handle this without making sure that mudgirl gets some counseling.
I believe that what’s going to be happening with you and mudgirl in the future is going to be very, very rough. It’s just better than the effects of doing nothing. Which is a damned cold comfort to me, right now.
On preview: Chimera, I don’t disagree with the position you’re taking, that mudgirl will assign an importance to this event based on the reactions of the adults around her to the incident. But I think you have grossly underestimated the amount of attention that will be focused on this incident simply by the nature of investigating and prosecution.