Is it normal for my teeth shade to be slightly darker on the sides versus the front?

You can anything after whitening. You may want to stay away from foods like red wine, coffee and tea which will stain your teeth again. You may also want to avoid very cold foods, as your teeth may be sensitive to cold foods after a whitening treatment. 

This is one of the issues you see with whitening strips. They don't always align perfectly. Just as the lip sides of the teeth end up one shade and the tongue sides another. With custom whitening trays and gel it's usually not a problem. It will probably fade enough that it won't be that noticeable.

If no enamel is on the teeth, your teeth will not get whiter. Dentin or root surface will not bleach, with bleaching products , or charcoal tooth paste, as a matter of fact it can cause sensitivity which you do not want.for permanent bleaching, you need to explore dental bonding or porcelain ...