Well no wonder I didn't see it! Thanks for finally explaining it - I would never have got it.
Unfortunately, since all the other line-drawing etc was to do with the lines only, I read the instruction as meaning that, within the 24 letters there would be three of the 'original' words, and the rest
of those 24 letters, would all be real words with two backwards etc. So I spent hours looking for four-letter words hidden in some strange way within any two columns, and of course failed to find anything! Surely the instruction could have said "and the rest
of the grid"?
It didn't help that I was utterly convinced that no Listener crossword would ever contain an unchecked letter that wasn't part of a clue. But since Mrs A deleted the clue for DOBBERS-IN, that was exactly what we were left with, so I assumed that the final highlighting would include that column at least, which would somehow verify the annoying N.
I think the "instructions" for various pieces of the tortuous Listener logic have been unnecessarily ambiguous of late, this being a prime example. Another example would be 4760, where it was never stated that "solvers should INSERT the phrase's initials" into the appropriate cell. I did so, but it was a 50/50 call whether to or not, as the instruction merely said the initials "must be" somewhere in a cell which we had been instructed to erase!
Hmph!