A good dietician will limit your fat intake, because that is where most of the calories come from and where cholesterol is. Sugar is also problematic but only in refined form, and in fat people. The theory being that it's fat that limits the response to sugars, I'm oversimplifying but this theory is at the heart of the sugar/fat debate. Dieticians aren't debating this, mostly health gurus and doctors online are. The research into health versus diet is very clear and very much understood. Less processed food, more whole foods, less animal products, more plants. (fruit, grains, legumes, vegetables, nuts and seeds.)
And yes I do agree that the availability of healthy food is very limited and that understanding how or even what a healthy diet is, especially from consuming social media, is hard. The food industry is a multi billion profit industry, there are a lot of conflicting interests. But heart associations, diabetes association, association of dieticians, world health organisations, and many more are screaming from the rooftops. So it's also not impossible to figure it out. But if you're left to the almighty wisdom of 'the market' you're fucked. Yes, it's hard to stay healthy and skinny.
Nah. It's more about inherenting eating habits than genes. Also it's much easier to gain weight if you've been obese during your youth. Eating is also an unhealthy coping mechanism for a lot of people. Genes is probably on fifth place if not lower.