You can use any command on an alpha channel that you would use on a layer. You can use levels on the gradient, hand retouch it with the brush tool, etc. Because my experience with Photoshop is based in graphic arts and drawing, I don't use adjustment layers per se, ever. I work on duplicates of the original layer, isolating whatever section I want to modify and working on just that area. I am very comfortable working on files with many layers because this allows me the option of restacking the layers in any order to achieve the best blends etc. I don't flatten the image until the last step, and I always burn a backup copy of the unflattenened image in case it needs to be re-edited.
Using gradients in masks take time to get use to,but they allow for very subltle corrections. As someone mentioned, with digital media, you only have to do the corrections once for an unlimited number of prints. So the time you spend is really relatively short compared to retouching prints manually.
I've saved your attached photos. I'll try making some adjustments and note the technical data for you over this weekend, if you wouldn't mind. You didn't mention the final resolution you want to output. The higher the resolution you work at originally, the more seamless the corrections you make can be as a whole, but the subtle details will get lost if you bump the res down alot. You might not even see them in the finished print. But you won't see any correction artifacts either. My commercial work is never less than 600pdi, even if my client only needs it at 300dpi.