I think the way the unclued answers are all connected by one of the "codes" is quite brilliant but I'm a bit puzzled over the "In the final grid all entries except the unclued ones are in a simple letter-for-letter code" - are we supposed to change all the clued answers in the grid to what will basically be gobbledegook?
... and aren't the unlued entries also in a letter-for-letter code - so why differentiate in the preamble?
I expect, as per usual, I'm missing something obvious as no one else has queried this but I've already gone back over it at least a dozen times so ...