Just to quickly update everyone. I received Baxter's biopsy results today. They came much faster than I expected. Overall, it is GREAT news. He is a Grade 2 with a mitotic index of less than 1. Grade 2 means "intermediately differentiated cells with potential for local invasion and moderate metastatic behavior." (wikipedia)
According to my vet and other canine articles, the mitotic index, "the ratio of the number of cells undergoing mitosis (cell division) to the number of cells not undergoing mitosis (in a population), is actually more important and predictive of survival of mast cell tumors of canines.
Since Baxter's was so low, it is a good prognosis for him. Had the mitotic value been 5 or higher, it would not have mattered what the grade was. So for now, no treatment is warranted since the tumor was fully excised.
This is all a great relief to me, as I was highly concerned Baxter might have a poor prognosis.
Thank you for all your support--those who commented, sent good vibes our way, or just thought of Baxter. It's times like these when you are reminded just how much your pet means, and that things can turn awry quickly.