The FIRST thing I would do is consult a lawyer. The facility that did the backwards MRI's should be held accountable for getting you the correct MRI results. That should include public relations with other drs/facilities. You should not be required to be trying to get other practitioners to help you out - the facility/staff at fault should be taking care of all of that for you.
Keep pushing on this issue, especially with help of legal counsel. Be very, very firm when dealing with the original facility. While it may not be their fault the machine was put together wrong, it IS their responsibility to ensure the accuracy of their diagnostic testing. This situation if not caught could have (but hopefully WON'T have) drastic consequences for you. Be matter of fact when dealing with other facilities/technicians/drs, but when dealing with the people who made the mistakes, be very, very firm - let them know that the situation had NOT been corrected to your satisfaction and to the degree of certainty that you need it to be, and that you will stand upon your rights to be given the proper services that you and/or your insurance co have paid for.
It would also be prudent to talk with the lawyer about exactly what your communications with the insurance co should be in this matter. If the original facility that was at fault is trying to get this all billed again to your insurance, that could be why the other facility only did an ultrasound. Any more diagnostics that you need to have done to get this straightened out should be billed to neither you nor the insurance company, but to the facility at fault.
Whether you confer with a family lawyer or an injury attorney is up to you - but I seriously would consider a consult with an injury attorney - many of those law offices give first consultations for free.
Sincerely,
Tre