I largely agree with this, except that I don't think it's productive (or scientific) to make general claims about biology or environment being "the primary influence on our behavior." The point of the OP is just one example of the infinitely complex interactions that occur between the biology a person is born with and the environmental context in which behaviors manifest. These make such assertions about % variance due to biology, including those based on twin studies, invalid because some of the variance that such studies would attribute to genes is more directly and proximally caused by environment. For example, studies on race and IQ. If IQ tracks with the child's skin color rather than which parent's raised them, then that variance gets attributed to genes. However, such studies do not control for the effects of living in a racist society or the long term cultural effects of historical slavery/racism. If two black twins are separated and raised by families of different SES that doesn't mean that any similarity in their IQs is biological. Because, if they still both live in a similarly racist culture, then this will cause their IQs to converge with each other and diverge from those of other races, leading to an overall correlation of IQ and "race" that is actually caused in part by cultural racism and how it connects biological skin color to intellectual development.
It is also important to note that because, as you point out, people evolved to be adaptive to culture and culture can and does change, that means that arguments that refer to what is "natural", "biological" or "evolutionarily adaptive" have little to no relevance to discussions about what kinds of cultural, moral, or legal norms we should create or change. As you said, selection pressures have wired people to conform to whatever the norms are and to adapt to norms, which includes to changing norms. So, outside of norms that are biologically impossible, we should ignore what is "biological" when setting or changing our culture, and create whatever norms cohere with our ethical principles and people will adapt to them.
As far as twin studies go, I believe most experts would agree that similarities between them are going to be mostly genetic, especially as it pertains to IQ. Essentially, their brains would be genetically equivalent, and only save some extreme type of socialization a lot of their traits would turn out the same. So socialization does play a part, but I think you're off base here and understating genetics. Chemically, twins are the same person, which has a much more profound influence than socialization, which has been borne out by twins raised in different environments (check out Google Scholar here.. or Pinker's The Blank Slate).
When I say biology is the primary influence on behavior I'm not just talking genetics, though, I'm also talking about the profound influence of biology on culture itself. It's a given that culture is plastic,
but culture is also a reflection of our biology and psychology. This is a critical point because if cultural realities ultimately originate from our physiology, then those influences are constrained by the same reality.
- How about a society where some form of partnering doesn't exist?
- A society that doesn't prize youth in females?
- A society without some form of religious or idealistic beliefs?
- A society where members don't compete for resources?
- A society where men aren't naturally aggressive
- etc
And so it's not just genetic behavior per se, but the reality of being human, in a human built society that dictates most of what we do. It's kind of a false dichotomy to contrast genetic/environmental influence, because our genetics build the environment in the first place.
In your second paragraph you mention we should change norms that aren't biologically impossible, so I think you're on base with this, but I'm more or less of the opinion that those mold-able parts of our culture are actually much more rigid than you, which seems to be the main difference between our arguments. As far as ethics go I don't know if you can consciously shape that, outside of the natural evolution of a community that's building scientific knowledge and increasingly protecting human rights. And even then a sizable component of any community are likely going to be self-serving, one way or another.