DDT doesn't make a very good repellant, But I think I get where you are coming from. It's why it's important to know at least some of the chemistry behind what you are wanting to use. When I was growing up you could still get DEET in 99.5% bottles. Turns out that it doesn't work any better than 24% after a couple of hours. The other main repellant Picaridin doesn't work as well, needs higher concentrations more often, and how it's metabolized in the body is less well known. Permethrin is actually broken down in the first layers of skin, so it doesn't work as a repellant directly on the skin, but keeps bugs from landing on you and biting through clothing, mostly via them being dead. It's used for treating certain things like scabies, is particularly toxic to cats, and is destroyed by UV. Sometimes the "worst" chemicals are also the ones we know the most about. And to your other point, if it's on your skin, it's in your body. There are a few exceptions to that rule, but it's better to presume it's true than guess it might not be.
I'm all in the mesh camp, but full "no-see-um" mesh in the tropics can still hold enough heat to be miserable, and it's not practical when walking through the Aussie bush. So as with most things, custom solutions all around.