About u’vchukoseihem, we’re not talking about drashos. The p’shat, according to everyone, is an issur based in an avodah zarah concern.
I’m not speaking Drashos, either. I’m talking about ramifications. It is put there as an introduction to arayos, which were common in the nations with which we had contact. However, the meaning of the words itself are not lost on us, and it means not to follow their customs. If not for Arayos the Torah would say it anyhow but not there. This is very common in the Torah.
Otherwise we wouldn’t be able to limit the issur. Because the other consideration would still apply.
The Torah says, Toeva. That is where Chazal learn it. In fact, perhaps it can be a Raya to the Rambam. If it were simply a Lav to prevent mingling and Arayos we wouldn’t need a Pasuk to tell us that it doesn’t apply to that which doesn’t promote mingling.
I implied this in my first post, and I agree.
I read your complete comment but wrote this too, because of a slight difference. You were saying that they in general they worshipped idols so that they can do as they please — no idol cares about your behaviour towards anyone. I was adding that the rites themselves were very often made of lust.