1. There are no such shadows in the official art. I know my sprites appear a little flatter than the official art, but that's because of my individual style *cough* and due to effectiveness. But yeah, the contrast between the base color and the darker color really isn't that big. I'll have to find some kind of solution...
2. While those shadows work well in a single image, they would have to be changed for every single pose. The Paper Mario style is made of individual body parts that are put together to form the character. For example, Mario consists of these parts. If I now added a shadow to his left hand like you suggest, it would work while he is holding it down. But as soon as the hand faces up (jumping) the shadow would look completely weird.
3. I simply don't have the time or motivation to redo everything I've already done
I hope you understand what I mean. I really appreciate your advice and comments.