I would have to see your teeth in person, but to answer your question-- Yes. There is usually a way to polish and smooth teeth to make them look and feel smooth and shiny again!
My guess is that the nerve of this tooth is dying. It probably has been changing color and your teeth whitening highlighted the discoloration. The lesion on the gum is probably a fistula. I would advise testing for vitality and possible Root Canal Therapy.
I agree with the other dentist, I also have not had much success trying to whiten a single dark tooth as a result of trauma. You may have to consider a veneer or crown to correct this condition. Good luck