I am sorry for the stress and concern that you are having. Mast Cell Tumors can be both frustrating and scary. The grading has to do with the level of aggression and they do not necessary progress through the grades of 1,2,3.
Unfortunately grade 2 MCT can vary widely - some seem to behave more like 1 and some more like 3. The histopathology (biopsy) report sometimes can help you figure how aggressive it is in the description.
I would recommend that you have an evaluation with an oncologist.
Most vets are familiar with MCT however when they are above a grade 1, having the involvement of an oncologist may be prudent.
I hope this helps.
Christopher A. Lee, D.V.M., C.V.L.S.
Promoting surgical options and pet comfort through the use of lasers.