That we can use alternate food sources in times of hardship is a survival mechanism. IOW, omnivory is not a preferred mode of eating (genetically/evolutionarily speaking).
Individually of course, we may prefer omnivory, frugivory, x-vory.
Anthropological research in conjunction with modern medical research convincingly indicate several foods cause disease. The fact that we can digest all kinds of things is irrelevant.
It pretty much all boils down to avoiding sugars, grain products, and certain oils. It's a very easy case to buy if you read into it.
The less verity in a persons diet the closer to perfection what they do eat needs to be. When you consider the quantity of fructose in the average American diet of course there is a problem, but mainly because of what it replaces and not what it is. If you burned 4k calories a day and drink 1k worth of sucrose water you can still have a healthy if unusual diet. However, when you are only consuming 2k worth of calories a day 50% of which provides zero nutrition your body is probably missing out on important things.
Yes, and there is a panoply of diseases only people eating grain get. They're referred to as the "diseases of civilization."
Do your own reading. I did mine and concluded bread intake should be minimized, whole grain or not.