... So, if I wanted to remove all selections first, then perform the code you have above, would that need to include:
(gimp-selection-none image) - ?
I may have missed something, but from my cursory examination of the (charcoal) script, there doesn't seem to be a selection active* at the time you perform your scaling. Performing a 'gimp-selection-none' would not hurt anything, but I didn't think it necessary (and if it were necessary then I should feel compelled to re-examine my understanding of what is supposed to happen).
* The preceding operations entail a Select By Color, followed by copy and paste. When the paste occurs (creating a floated selection layer), there is no longer any selection present (even though the display may show marching ants around the original selection), and there is still no selection present after this floated layer is anchored.