Kt17, I enjoyed hearing your view of this puzzle. I don't think you enjoyed it as much as I did!
I think most solvers would have worked out how to alter the special answers long, long before thinking about piecing together the extra letters. There were obvious points of entry - solvers would have had correct answers that wouldn't fit, and also grid entries beginning with letters that didn't match up with any clues. The puzzle title helped too.
Life may be too short to want to complete that challenge, getting the initial letters ...but the challenge wasn't necessary at that point. The 'instruction' didn't help at all with any changes - though we solvers didn't know that until we'd filled the grid.
I enjoyed slotting all the entries together, it was very satisfying. I ground to a halt when it came to carrying out the instruction, but that was because I'd failed to interpret it properly! I liked the final step when I got there - eventually.