Hi OptO,
The thematic words in the across answers relate to a word in the title of a film by the name derived from the across misprints. The replacement letters are N, R, M and D.
The thematic words in the down answers are all versions of the last word in a famous work by man derived from the misprints in 1d but have to be read in a way described by the first word of the same work.
The replacement letters are U, I, B and N.