I'm not sure what you goal is (mind explaining a little more what you need to do?), but here's something you can try from what I think your goal is.
Here's your normal set up as of now:
- Two images (one overexposed and one underexposed)
- The top image has a layer mask. In your post you have make adjustments to the layer mask selection itself without it effecting the entire layer image
To do that you can just make the Layer Mask a selection in your image by right clicking over the layer mask and choosing Add Mask to Selection (or using a cool shortcut, in Windows it's by pressing Ctrl key and clicking on layer mask).
The result is:
Now any changes you apply to the image will only affect the part that is selected.
I don't really know if this is what you want because even if you did make adjustment to the whole image, it would be ok because the layer mask is only going to show the part you want. In my example, the girl, in yours I think it's the background.
Hope this helps set you in the right path.