Nerve pain flossing root canaled/temp tooth - ill fitted or periodontal issue? Need 2nd opinion. See a Periodontist, Prosthodontist or Dentist?

See an endodontist. With proper care, even teeth that have had old root canal treatment can last a lifetime. An additional endodontic procedure may be able to support healing and save your tooth. If you have pain or discomfort in a previously treated tooth, talk to an endodontist about re-treatment.
Return to DDS who pe. I would recommend returning to DDS who performed initial treatment. Any of the dentist you've listed would be able to give second opinion. The problem could be combination of ill fitting crown, periodontal dz. or other issue.
Nerve Pain. If the tooth had root canal therapy, then it should not be hurting while flossing. The gum tissue may be inflamed from the temporary, which happens from time to time. It might be wise to place the final restoration with temporary cement, see how you feel, and then cement it permanently if all is well.
Finish procedure. Is the root canal finished? See the dentist and finish the procedure. If problematic, have your dentist e-mail a radiograph to the dentist of your choice.
Likely temporary. For second opinion see Prosthodontist. If needed you will be referred to Perio. Good Luck.