Dear God please tell me if this is even possible.

[rich2005]

Gave up (1) spliting into columns and scaling using scripts (2) the Gmic plugin cartesian deform (must be possible but I can not figure out the correct equations)

Not Gimp, ImageMagick, https://imagemagick.org/  and from this page https://im.snibgo.com/sh2sh.htm

Code:

magick  in.png -fx "p{j==0?0:i*h/j,j}"  out.png

gives this before / after

   

It does require a trim along the diagonal.