I would put zero faith in any advice from a staffer at a district office about pretty much anything. As you have seen, the bottom line is that it's a pretty major distinction between federal and state and the notion that a federal court could expunge a state arrest record is fairly clearly off. This is not a political, but a practical observation. If you need the mailing address of a federal agency and cannot find it online yourself or you need help obtaining a form, those staff handling calls may be helpful.

Your problem is not likely the underlying facts, e.g. abusive parents calling the police, but the fact that the records are incomplete and do not clearly demonstrate that the matter is closed. Local court with local cops at some time in the past is not something which gets a lot of attention paid to it.

You might consider going to whatever court it was and seeing if the docket really does show that the matter is terminated. If it is, spend the money to get a couple of attested (certified) copies of the docket sheet showing that. If not, then you have identified the problem and can either work on getting that taken care of or finding a local lawyer who practices in that court who can figure out how things are done in that particular court.