Finally completed this one, well almost. I believe I have everything required to complete the final step "..... a unique solution guided by the original contents of the affected cells (among others)."
Trying not to give spoilers, I know where the 9 outcomes from the first instruction are, and what their original contents are. I know (from previous experience) exactly how to comply with the second instruction. I have made some fresh 'instructions' from the original contents of those cells.... a horizontal word along the top, 2 words along the diagonal, and another word along the bottom horizontal. Four letters aren't used - is that relevant?
There are 4 possible, symmetrical, solutions to this so my question is how to fix the 'unique solution'. I can see another 3 letter word that could possibly indicate the end of the solution. Is it this that fixes the orientation or am I missing something? Also, is there anything more to the 4 word phrase, from the letters of the original contents of the affected cells, than the 2 obvious interpretations?