Is it a reason or seperate mitzva? - "umal'ah ha'aretz zimah"

This is your SH”M bonus week. “Al techalel et bitcha l’haznota, u’mal’ah ha’aretz zimah”. Rashi explains that when a father encourages his daughter in znus (the first half of the pasuk), then the fruit of the land will become destroyed in punishment (the second half of the pasuk. The Rambam in his fifth shoresh sets out as a guideline that one should count only a lav, but not the reason for the lav, as a mitzvah. One of the Rambam’s examples is this pasuk: the issur is “al techalel et bitcha”, while “umal’ah ha’aretz zimah” is just a reason that gives added weight. The Ramban on chumah and in his gloss to SH”M takes issue with this approach and writes (to defened the BH”G’s count) that the end of the pasuk is actually a separate lav that applies to the zonah and her partner – how, asks the Ramban, would we derive such as an issur from the first half of the pasuk, which addresses itself only to the father? The achronim point out that our pasuk is actually part of a broader issur of “lo t’heye kedeisha”, which prohibits a man or woman from having relations without kesubah and kiddushin (see SH”M lav 365; the Ramban there defines the issur znus differently, but that is another discussion). The Rambam subsumes our pasuk under that general issur, but does not count it as a separate lav, and interestingly the Ramban in his gloss there notes his agreement with the Rambam’s understanding! Meaning, (I think), that Ramban concurs with the Rambam that the seifa of the pasuk is not just an aggadic addition (as Rashi understood), but has halachic significance - the question is whether it is a mitzvah (BH"G) or just an offshoot din d'oraysa.