navi sheker -- issur gavra or cheftza

There is a din that a navi cannot add any mitzvos to the Torah.  Therefore, if a navi tells us that G-d commanded a new mitzvah, we know he is lying and we should not believe him. Ramban has a safeik whether such a navi would be chayav misah as a navi sheker or not (13:4):

ונראה, שאם מתנבא בשם י״י לצוות מצוה שיחדש, כגון שהיה במקרא מגלה (בבלי מגילה י״ד.) שאינו חייב מיתה, אלא שאין לנו לשמוע אליו, דכתיב: אלה המצות (ויקרא כ״ז:ל״ד) – אין נביא רשאי לחדש בהן דבר מעתה. ואולי כיון שלא נאמין לו, הנה הוא נביא השקר ומיתתו בחנק.

What's the safeik?  If the navi is telling us nevuah that cannot possibly be the dvar Hashem, why would he not be chayav as a navi sheker?

In next week's parsha, where the Torah spells out the onesh misah for the navi sheker, the Torah tells us (18:21-22):

וְכִ֥י תֹאמַ֖ר בִּלְבָבֶ֑ךָ אֵיכָה֙ נֵדַ֣ע אֶת־הַדָּבָ֔ר אֲשֶׁ֥ר לֹא־דִבְּר֖וֹ ה׳

 אֲשֶׁר֩ יְדַבֵּ֨ר הַנָּבִ֜יא בְּשֵׁ֣ם ה׳  וְלֹֽא־יִהְיֶ֤ה הַדָּבָר֙ וְלֹ֣א יָבֹ֔א ה֣וּא הַדָּבָ֔ר אֲשֶׁ֥ר לֹא־דִבְּר֖וֹ ה׳ בְּזָדוֹן֙ דִּבְּר֣וֹ הַנָּבִ֔יא לֹ֥א תָג֖וּר מִמֶּֽנּוּ׃

How do we know whether the navi is a liar or not?  The Torah tells us that the navi will give a sign or a prophecy that will fail to come true and that will give the game away.

R' Gifter quotes R' Ahron Shechter, R"Y of Chaim Berlin, as explaining that this is the Ramban's safeik.  Maybe the navi is chayav misah only where you can point to empirical evidence that proves he is a liar -- the sign he promised failed to materialize; the miracle he promised failed to happen.  But maybe in a case where the navi invents a mitzvah, where it's only because we have a din that tells us that this is something a navi cannot do that we know he is lying, he would not be chayav.

I would like to suggest another sevara.  Minchas Chinuch asks why any time a person transgresses any issur he/she is not also liable for the issur of violating the words of a navi, given that Moshe Rabeinu was a navi.  R' Soloveitchik answered that there is a difference between the cheftzah of nevuah and the cheftza of Torah.  Moshe the gavra was a navi, but the commands he gave (or at least 613 of them) have the status of Torah, not nevuah; they are qualitatively in a different, unique category unlike other prophecies (see this post.)

When a navi tries to introduce a new mitzvah, he altering the cheftza shel Torah, not just saying false words of prophecy.  Perhaps the safeik of Ramban is whether the issur of navi sheker is an issur on the gavra, in which case no matter what the prophecy is, the navi is chayav, or whether the issur navi sheker is one of presenting a false cheftza shel nevuah, in which case changing the cheftza shel Torah is a different matter entirely.