An interesting script but have you considered adding an undo feature to it so that the effects of changes to the control can be evaluated more easily (by just pressing Ctrl-Z rather than having to delete the 2 layers the script creates)?
Changing the start of the code to read:
(let* ((layer-new 0)(layer-new2 0)(layer-new3 0)(mask-new 0)(curve-new 0))
(set! layer-new2 (car (gimp-layer-copy drawable 1)))
and the end of the code to read:
Seen it now... I wasn't quite familiar with the question system. Good idea, thanks for the suggestion! I will upload an update in the following days.