Time to start working on recovering information from the sky. The result of the 3-tone process is the input image. Restore the SkyMask with the mask tool (mask, combine). Then blur the sky mask with the mask/blur tool using R=80. Restore the sharp edges by selecting the "overlap" option on the mask tool (rest your mouse over the icon and it will say "Overlap", third one from the left) and combine again with the origina SkyMask.jpg (use "Apply" at the lower left). The resulting mask is the one to use with the composite transform to paste in the better sky image. One further reflection, I think I set the blur radius too large, so if I were to do this over again, I would try some smaller values.