R' Aharon Lichtenstein zt"l was asked who he would rush to meet first when he got to the olam ha'emes. Who among the pantheon of Rishonim does he admire most? You would think someone steeped in Brisker lomdus like R' Aharon would of course answer "the Rambam," but guess again -- it was Ramban who most fascinated R' Aharon (see the article I linked to). Parshanut, philosophy, mysticism, halacha -- Ramban touches on it all. I want to look at one issue related to our parsha that brings that multi-faceted outlook into sharper focus.
A Jew must sacrifice his life rather than violate any one of the three cardinal sins of avodah zarah, arayos, or murder. However, when it comes to other prohibitions, the Torah tells us "v'chay bahem." It's not clear whether that is simply a matir, a license to avoid the sacrifice of mesirus nefesh, or whether it is a commandment, an order to preserve one's life even at the cost of violating other prohibitions. Tos (Avodah Zara 27b) would presumably take the former position, as they hold that a person can choose, if they so desire, to give up their life for any commandment. The Rambam, on the other hand, holds that one is not allowed to give up one's life unless obligated by halacha to do so, and so presumably he would take the latter position.
The Avnei Nezer (Choshen Mishpat 193) has a fascinating teshuvah in which he discusses whether a person suffering a life threatening illness who is told by doctors that he/she must eat on Yom Kippur is permitted to forgo medical advice and fast anyway. Rather than approach the issue from the perspective of the different viewpoints of Rambam/Tos above, he instead cites at length the Ramban on Parshas Bechukosai who argues that consulting doctors is only for those who are not on the ideal level of bitachon, for those who do not understand that everything is in G-d's hands alone and that illness can be cured by teshuvah. "Mah cheilek b'rofim b'beis osim ratzon Hashem!" Who needs doctors when you have G-d? Most of us are not on that level, so the Torah allows us to live b'derech ha'teva and get medical help for our problems. However, for a person who truly places his trust in Hashem, whatever the doctor says is irrelevant.
R' Ovadya Yosef (Yechaveh Da'at 1:61) discusses this same issue and interestingly, he also cites a Ramban: Ramban in the Milchamos in Sanhedrin (74) writes that not only is it not a midas chassidus for a seriously ill person to fast on Yom Kippur contrary to medical advice, but to the contrary -- a person who does not eat when there is danger in not doing so is liable for taking his own life! (Just for the record, at the end of the section on Moadim in the Shem m'Shmuel there is a letter from the author, the Avnei Nezer's son, to someone in the hospital over Y"K in which he warns the person that they must eat if instructed to do so by doctors. Did he backtrack from his father's position?)
What would Ramban the halakhist writing the Milchamos say to Ramban the philosopher/mystic's argument in his peirush al haTorah? Why is it not a midas chasidus to fast if a person trusts fully in G-d? Will the "real" Ramban please step forward and make his views clear?
Obviously both Rambans are the "real" Ramban. Somehow Ramban the philosopher/mystic saw no contradiction between what he wrote in his peirush al haTorah and what he wrote in the Milchamos. If we only had one or the other, it would be easy for us. But the greatness of Ramban is that he gives us both -- an abundance of riches! -- and leaves it to us to puzzle out how to fit the pieces together. I'll leave it to you to do that : )