When I was learning this Ramban with my daughter on Shabbos, we defended Rashi by suggesting that the Toras Kohanim is a derash, but Rashi is concerned with peshuto shel mikra. Rashi’s agenda here is to explain based on peshuto shel mikra the connection between “kedoshim” and the previous parsha that dealt with arayos. The Toras Kohanim’s concern is developing through halachic derush the moral/halachic parameters of what the mitzvah of kedoshim entails. Assuming this understanding of Rashi is correct, it begs the larger question of when (or even whether) Rashi concerns himself with explaining smichus haparshiyos and when he does not. He does not, for example, try to piece together connections between the many mitzvos that open parshas kedoshim (as other meforshim do). Be that as it may, what I still do not understand is if indeed Rashi is concerned only with the issue of smichus haparshiyos, why does he not just say kedoshim means “persuhim min ha’arayos” and leave it at that? Why does he add the additional words “u’min ha’aveira?”
2) On a different note: Aside from the categories of chovas hayachid and chovas hatzibur, mitzvos incumbent upon the individual and mitzvos incumbent upon the community as a whole, there exists a third category of mitzvos: a chovas ha’yachid that can be fulfilled only b’tzibur. For example, the Ramban (Milchamos to first perek of Megilla) writes that although both the reading of megillah and the reading of the Torah must be done b’tzibur, there is a big difference between them. Kir’as haTorah is incumbent on the tzibur as a whole; kri’as hamegillah is incumbent on the individual, but the reading done by the individual must take place in the presence of a tzibur. (See this post).
Chazal tell us that the mitzvah of kedoshim tehi’yu was said at a gathering of all of Klal Yisrael. I would like to suggest that while there exists an obligation on every individual to try to infuse his/her life with kedusha, the Torah here is teaching that this mitzvah can only be fully achieved in the context of community – it is a chovas ha’yachid that must be done b’tzibur. The hava amina of the Toras Kohanim, “yachol kamoni,” means to suggest that one might have thought that one can achieve kedusha by living in isolation; kah mashma lan the Torah Kohanim that this is impossible. The path to kedusha lies through interaction with the community, growing with them, feeding off their energy, and contributing back to their growth and improvement.