That's a BS response they gave you. NBC does not schedule the interview on behalf of the FOs. Each FO is responsible for scheduling their own interview. The Chicago Lockbox receives the AOS package, checks for completion (as in all forms signed as required, correct amount of payment enclosed, priority date, etc), scans the documents into the system following which they attach the applicable cover page to the case and forwards the package to the NBC. NBC separates the I-485 from the rest of the package (assuming the package also includes an I-765 or I-131), schedules the bio appointment and sends the I-485 form with all the supporting documents to the applicable FO, they do not wait for the background check to clear before forwarding what needs to go on to the FO - and this explains why some people have reported receiving IL even before they've gone for their bio appointment.
I don't doubt the fact that your background check is still pending and your FO is reluctant to schedule an interview without it. And it's possible that's the file they're still waiting on. Definitely not the AOS file itself.
Anyway, I suggest you wait for now for a response regarding the service requests you've raised, both personally and on your behalf. Depending on the outcome of the service request, you may need to request your congress person to initiate a follow up request on your behalf.