I would go see the nurse practioner .. and tell her of your concerns. That you are not clear what, if any, diagnosis you have (because your records were lost? re-created? some confusion between doctors?). SHE might not have lost your chart -- the office staff might have, and are covering their butt. It would be pretty unusual for a doctor to lose the chart -- but she passes it back to the staff who are supposed to file it away. I suspect they've been scrambling to see where they mis-filed it!
If you're looking for a doctor who will give you an instant diagnosis -- that's not likely, until there is some clarity in your test results: enough tests coming back "positive for MS", for a doctor to declare that. And possibly that has happened with you, but that message hasn't been clearly transmitted to you!
Of course you can see another doctor..but this is doctor number 3, they've all had relatively the same answer: "We don't know what's wrong; we need to watch and wait and see what develops", and seeing another doctor may just add to the confusion.
Nurse practioners are pretty highly skilled. And usually have more time to meet with a patient, and answer questions and concerns. I wouldn't have any hesitancy myself to be seen by one, rather than a doctor, for follow-up visits.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....