"And the man who shall do": (A rebellious elder is not liable) until he teaches to do (what is contrary to the ruling of the Sanhedrin, [but not if he only voices disagreement with their ruling]).
"wilfully": He (the rebellious elder) is liable only for witting and not for unwitting transgression.
"not to listen to the Cohein": (directly), but not ("not to listen") to one who hears it from him.
"to the Cohein who stands to serve": (Ibid. 18:5) "to stand to serve" tells me that the service (of the Cohein) is only "standing." This tells me only that it is a mitzvah (for him to stand). Whence do I derive that it is a categorical requirement (i.e., that it is not valid unless he is standing)? "who stands to serve" implies that if he sits, his service is invalid.