Getting back to the debate between the Rambam and Ramban: The Ramban asked, if all Rabbinic enactments must be obeyed because of lo tasur, then why are there leniencies for dinei derabbanan and not dinei d'oraysa? Why do we say seifeika derabbanan l'kula but sfeika d'oraysa l'chumra - every safek derabbanan is also a safek violation of the d'oraysa of lo tasur?
R’ Soloveitchik in Shiurium lZecher Aba Mari (‘Two Categories of Mesorah’) addresses himself to the Rambam, and although he does not directly offer answers to the Ramban’s questions, applying his sevara seems to resolve them. R’ Soloveitchik writes that it would be absurd to suggest that someone learning Masechet Megilla, which is devoted almost completely to dinei derabbanan, has not fulfilled a mitzvah d’oraysa of Talmud Torah. Although the obligation on the gavra to fulfill the mitzvah of megillah is only derabbanan, the laws themselves that govern its fulfillment become part of the corpus of Torah law, the cheftza shel Torah. Just as the study of Rabbinic law is a fulfillment of the din d’oraysa of Talmud Torah because these laws have become part of the cheftza shel Torah, the zakein mamre is liable for contradicting these laws because he has nullified a portion of the fabric of Torah law.
The Ramban’s questions focus exclusively on differences between the level of obligation of the gavra, the individual performing mitzvos, with respect to dinei d’oraysa and dinei derabbanan. The Rambam need not take issue with those distinctions at all. The Rambam’s chiddush viz. the zakein mamre relates only to dinei derabbanan as a cheilek of hora’ah and cheftza shel Torah, not to personal obligation.
R’ Elchanan cites R’ Chaim Brikser as explaining that the Ramban too held that the license to create dinei derabbanan stems from lo tasur, but only in the overarching sense – one cannot point to a particular din derabbanan and say the Torah prohibited that action; merely that the Torah licensed Chazal to legislate as they see fit (Bill Selliger beat me to this in the comments). This does away with the need for the mystical notion of ratzon Hashem as an independent mechyeiv, but, as REW points out, brings us back to the same questions – according to the Ramban, why do we not treat every safeik derabbanan as a safeik d’oraysa for violating the license of lo tasur given to Chazal? The answer would seem to be that without proof of issur, acting in a doubtful circumstance does not itself constitute a rebellion against the authority of Chazal (see also Meshech Chochma P’ Shoftim on lo tasur, also see REW who cites a different explanation in the name of R’ Chaim for the Rambam).