Finally got there after seeing three words emerge from the end of my initial letter pairs, then consulting the internet for further precise information. Even then I had to make many changes to my initial letter replacements, as Ionacarr predicted (and thanks Ionacarr for encouraging my persistence).
I found the endgame, and the final grid, rather unsatisfying. Given the word that needs highlighting, the orientation of the central column is 180 degrees out. It would have been better reading bottom up.
I'm also find the ambiguity of what to submit for the final grid a bit unsatisfactory. I can see an argument for both options, but in one case I would not be demonstrating that I have fully understood the method of entry of some answers nor the need for two other changes. It could also be argued that the preamble phrase, "cause of the coincidence," implies everything needs to be transparently indicated, so I'm going for transparency.