I actually did it!
Turns out that if you create the kind of combined rotation matrix I used in the script, if you do the multiplications of the individual rotation matrices in "the wrong" order you get different results that do the rotations in different orders. So if you multiply Yaw and Pitch and then Roll you get the "reversion" matrix to the one I used in the script, and I was able to pretty easily plug it in... Except it took me half an hour figuring out the exact if-statement syntax in the script, before noticing I should have been trying to figure out the
other kind of if-statement syntax, half an hour more figuring out the other kind of if-syntax again and after debugging by putting minus signs in front of every single alpha, beta and gamma in the else-branch formulas, bob's your uncle!
I left a gui toggle in the Rotate Equirectangular Map script only, because the other scripts don't really need this functionality that much. With the same values (do not turn the angles negative) just click on Revert and you should get the same rotations as before the first transformation. Of course the distortions on the edges are
really bad, but that's life.
https://pastebin.com/NKN9VEE4