Right now Buff chooses between row vs column when one square is selected based on which has more squares. That is better in most situations, but not if the spell backfires because there are too many squares. Might I suggest Buff selecting the row/column with fewer squares when the option with more squares would cause a backfire.
I’m really liking your game so far. I’m amazed how much depth you’ve put into a picross RPG, especially how even if you know the solution to the picture you still have to plan your moves out in advance. The way you use the grid as a resource to be managed feels truly original and fresh for the genre.