"Can her mom legally keep her daughter out of it since she's still a minor? (I'm assuming her mom will give her own deposition.)"
No. But the daughter can refuse to talk without a parent, and the mother could have told the daughter not to talk to the police. This would basically just be a refusal to cooperate. The mother would have spousal immunity, so she couldn't be forced to give a statement. In times like these, the police may threaten to arrest her under the guise that they don't know who downloaded the images, basically forcing the wife/mother to give a statement or go to jail. But police can talk to a minor as long as the minor doesn't state that she wants her parents there, especially in cases where the minor is a potential victim of parents. BotXXXXX XXXXXne, the mom wouldn't be able to force the police to stay away from the daughter, but the daughter might be unwilling to help and there is not much they can do about that.
"Would an attorney try to set up a meeting like this, or would he go straight to subpoena/deposition?"
a prosecutor may try that... but again, the flaw is that they can force her to give a statement even as a minor. So nothing would change when she became 18. More likely, she refused under her mother's influence, and now that she is out on her own, the prosecutor thinks maybe she is more open to it.
"Is that scenario totally unrealistic? If it could happen in the way I've described, would the prosecutor have the investigating law enforcement officer there to help him show her the comics?"
yes, it is somewhat realistic. Certainly realistic enough for legal fiction. These things vary from person to person. Under your scenario, I could believe that a prosecutor would try to turn a witness without police presence. Until now, she has been uncooperative, he is not really deposing her, he is trying to get her to cooperate, which can be done without the police.
"Q: And if it's not realistic-if I have to have the prosecutor call her in for a deposition--would she need a lawyer present? Could her mother be in the room with her (even though she's 18 now)?"
She would have a lawyer only if she brought one. Her mother would not be allowed in if any party did not want her there (including the prosecutor). But as I stated, I think that if the issue is that the prosecutor is trying to get Tera to cooperate with the State, there is no need for a deposition, and it is believable.