Well, I am a mathematician and it never occurred to me that I should be working modulo 27 rather than 26, although I should have read the preamble more closely. I think I have the thematic writer, but they do not seem to relate to any of the ten extra words. I am puzzled by the blank but perhaps it will dawn on me. The message from the across surplus letters does not make a lot of sense, though that from the down letters does. I think I am going to be in for a lot of grid staring!
Minor point: If 4d is &lit, it seems to be a strange description of the tragedy!