I can see where the doctors are split. Chemo is recommended for adverse features, and according to your information, there are two, one is the pN5 nodal disease, two above the N3 adverse features, and other is one being in level IV in the cervical neck, unless they re-staged after surgery to N0. Usually oral cancer is mainly in nodes in levels I-lll, if it metastasizes, and not as much in level IV a small percentage.
Good luck