Well, the faddists and media hogs are mostly just taking what mainstream nutritionists say and running wild with it.
I think you are confusing the educated nutritionists with people who get their wisdom from the media.
Granted, in an academic environment, I have ready access to actual educated nutritionists, I've discussed it with them. Their advice never changed. Everything in moderation, and moderation means limit sugar intake, moderation in simple carb intake, complex carbs are preferred (think cruciferous veggies and similar) fat in moderation, but more than so pop culture experts recommended - which was severe limitations on fat. Caloric intake related to exercise.
Since I played Ice Hockey for many years, it was sometimes difficult to keep weight on. The nutritionist told that I could do 4000 calories a day, and when I retired, to be careful about the simple carbs, as transitioning to less activity could be a trap in re-regulating my appetite. So I'm still active, but it was tough re-regulating. about 2800 calories per day is right now.
Are eggs good or bad? They were good, then they were bad because of the cholesterol, then good again because even though it's "bad" cholesterol, it's somehow different when in an egg.
The nutritionists I worked with were very bothered by the pop culture advice to severely restrict egg intake. Eggs are a really inexpensive source of protein that doesn't add a lot of calories - around 70 calories per egg. They were concerned that the elimination of eggs from poor people's diets, which were already laden with cheap carbs, was going to just add to the obesity problems of the poor, who would just replace the eggs with more cheap carbs.
I never stopped eating eggs, ate around one every day when averaged out.
Is too much salt bad, or do you just need to drink more water?
Ya gotta define too much. If you are retaining water, or are personally sensitive to salt, you cut back. OTOH, my hockey days caused me to need to use a lot of salt and potassium. I'd sweat off between 10 and 15 pounds of water a game - not an exaggeration. So even before I'd take a drink of water, I'd have some potato chips, then a potassium tablet. I dunno about normal people, but I can sense when my salt input is right.
Hell, I remember when olive oil was supposed to be really bad for you, just a heart attack in a bottle, so trans-fats were the way to go.
So, I say ignore the "mainstream nutritionists" as well. They're just going to reverse course in a few years, so eat like your grandparents did and leave it at that. Or great grandparents if you're under 30, I guess.
You see, those people telling you things and waffling back and forth are the pop culture nutritionists I speak of. The people who tell you that everything you are eating is wrong. I can find people who say that a complete Vegan diet provides every nutriment a person needs. It can't. I can still find people that say eggs are the worst food a human can eat. I can find people who say that you are going to die soon if you eat any cured meat, because nitrates are poison. Yet they regularly eat vegetables high in nitrates - and indeed food like "uncured" bacon is a lie. It is cured with celery juice, which is high in nitrates.
My nutritionists roll their eyes when they hear that. It's cured bacon - you have to cure that type of meat because botulism is never going to be a fad.
Now there is definitely some wisdom in your suggestion to eat like your parents or grandparents did. We have the advantage of having good nutritious food all year. Years ago, they ate cured and otherwise preserved food during the winter, and went big time for the fresh stuff in the spring out of boredom. So we can even improve on their diets without fads.
