ShepFl,
I have pretty much the same situation and have controlled Gallberry in my "Super Slash" with mowing. I did use Arsenal for injection to eliminate Water Oak, Lobolly Bay, Black Gum, and Holly to open up the floor for sunlight and pine seedlings.
I attended some seminars several years ago on Arsenal and was told that it does not effect pines, nor does it move in the environment. It works by inhibiting a protein used by hardwoods for processing sugars and causes them to eventually starve from lack of nutrients. It is slow but thorough and safe to humans. Even though I used it for injection, it is supposed to be good for spray and is used for "Release" of pines.
Fire is also a good herbacide and, when used regularly will keep the understory to a minimum.