I've made some progress on this -- I've solved all but one clue, and I have located and shaded the speaker of the second quotation (the one from the extra letters) in the grid.
However, I haven't been able to solve 6D. I also haven't guessed the first quotation, nor have I found its speaker in the grid. And thus I still have 33 unfilled squares in the grid (due mostly to the uncertainties caused by having to "treat" thirteen answers).
I'd appreciate a hint on 6D.