Make sure the
layer is active, because when you work in the Channels list, this make the channel the active drawable and thus the target of the ensuing Curves/Levels/B-C operation. So you have to select your layer again...
As you would expectt, a good deal of this
has been scripted.
Hi, there various ways to improve an image in GIMP. I recommend taking a look at this
tutorial which focuses on the different tools available in GIMP.
Since you are targeting the shadows and highlights, you should use the Dodge/Burn tool, a
luminosity mask, the Curves tool and also the Levels tool 
@Kriustal279
It seems you like the edits101 web site. Why not just post a single thread in the tutorials section.
For this one, well the dodge / burn / sharpen tools are somewhat of a legacy from days gone-by. There are better options these days.