I'm puzzled by piffleworthy's comment at #70: "There are four 'qualifying' columns. It's the next to last."
I assume he means it's the next to last of the four, not next to last in the grid (ie. col. 11).
I can see only three qualifying columns, i.e. columns which have a repeated letter in unchecked cells, AND where changes would result in real words.
So far none of the combinations of changed words has yielded anything in Google, nor has what I have found in the SW quadrant.