Been taking so long to get this finished, you deserve a peek at where I am so far. I just now found that MyPaint has a shortcut for flipping the view—just what I need to check for unintentional skewing! See some of my recent posts about my struggling with just this problem.
MyPaint’s flip view is intriguing in its operation, because it does not actually flip the art. That is, the file does not save one way or another depending on the view. And yet, you can operate on the art in the reversed view.
Since I somehow missed this function before, I force-reversed this page in Gimp, and have been re-pencilling it backwards. To add the dialogue to the reversed image in MyPaint, I tapped ‘i’ to for the flipped view. But as far as MyPaint is concerned, the file still goes the other direction, and exporting it to JPG gave me a backwards image, which I then had to flip again in Gimp.
So in future, I can save time by skipping Gimp and leaving the image facing the right direction, and just flip at will in order to draw and check my work to avoid accidental skewing.