Plato, Aristotle, Descartes, Hume, Locke and on and on. Long before the internet or even electricity people have studied the question of truth. They haven't found the answer, but they have been able to categorize and organize the questions. Again, I strongly urge you to pick up some of those books and read.

Let's start with your first question, about diet. You're conflating "best" in a way I believe both sides of that argument would not agree with. If you can't ask the right question how can you get a good answer?

Those who argue the carnivore diet is best for humans do so because humans have been eating animals from the beginning and because certain proteins, that humans need, are best acquired from other animals (or so the argument goes). The question is what's best for an individual's health.

Those who argue for a vegan diet generally don't do so for the sake of an individual's health but the life (spirit) of animals and our collective need for human survival. Meats generally take 10x the energy to produce. If mankind ate mostly vegatarian then we'd have less fighting over wealth (meat). However, vegatarians will be the first to say that one has to be careful if they don't eat meat that they get certain nutrients from certain vegatarian foods.

So the way you framed the question doesn't work for me because you're mixing apples and oranges, so to speak.

Educated people (and I don't mean people with diplomas, but people who read books like the above) don't believe the questions you've listed are good questions. Bad question, bad answer. Good question, better answer.

The problem isn't that we don't know the truth. The problem is most of us won't spend the time learning what others have said on the subject who have spent their lives studying it.