I don't think that the scientific consensus on nutrition as unsettled as you say. As an example, there are a lot of people making money selling various diets as well as promoting uncertainty and doubt around the issue, but there seems to be pretty definitive evidence in favor of cholesterol and fat increasing cardiovascular disease risk. People love to misrepresent studies, or cherry-pick poorly designed studies, and use them to claim that the consensus is wrong.

I'm not talking about observational studies either, but actual controlled feeding trials where they put you on a strictly controlled diet for a period of time.

Even if you look at what humans are evolved to eat -- evolution puts selective pressure on reproductive fitness. As a process, it does not put any pressure on you to live a long time, as long as you reproduce successfully (which is why insects like the mayfly can even exist). So looking at what primitive people ate does not really give us information about what is healthy if you want to live a long time (aside from avoiding things that are obviously immediately poisonous).

Even hunter gatherer tribes that eat a meat-and-dairy-heavy diet like the Maasai have been examined and have pretty significant cardiovascular disease -- but they are also so ridiculously active their blood vessels are much wider than people with a modern sedentary lifestyle, and that mostly balances out the narrowing from arterial plaque. Native people who eat a traditional diet heavy in whole grains, legumes, and tubers for calories have them beat by a mile when it comes to arterial health.