I did this to keep from needing to rewrite all the info in a comment. It contains all the information you've asked for here.
As for fiber, yes--it does feed gut microbiome. But just like us, gut microbes can eat other things. Nutrition science does not consider fiber an essential part of the human diet. If it did, carbohydrate would not be considered non-essential. And it's not that I have strong opinions about this--it's that I'm laying out the facts.
Ketosis isn't a shift in how the body uses energy--it's a shift in what the body uses to produce the type of energy it needs to do its work. If the fact that carbohydrate is a non-essential nutrient for humans isn't enough evidence that a very-low-carb diet is safe, I don't know what would convince you! You are basically saying that you question a diet that has very low amounts of something we don't need any of.
And, if one has a working pancreas, the threat of acidic blood (which is referred to as diabetic ketoacidosis) is non-existent. It is true that if one has existing kidney and/or liver issues, a very-low-carb diet can make things worse. But barring those pre-existing conditions, a very-low-carb diet actually improves kidney and liver function--it's the high-carb diet that causes the problems.
There is no research showing that the mitochondria in our bodies prefer and particular substrate for the production of ATP. How could there be? Are you suggesting that out mitochondria can express an opinion? Sure--there are some cells in the body that require glucose--namely the red blood cells. And while the brain will use glucose, there is no evidence that the brain needs glucose. We do know that the brain can metabolize ketone bodies--and when it does, there is less inflammation as a result. If mitochondria could express a preference, one would be hard-pressed to not think they would prefer the nutrient substrate that caused less inflammation...
If you look at the order in which the body will use nutrient substrates (which, by the way, doesn't indicate a preference), you'll see that it will use the most toxic substance in the blood before using anything else. So, if you have alcohol, glucose and fatty acids in your blood, the body will first metabolize alcohol, then glucose and finally fatty acids (and ketone bodies). In addition to this order of use, a high-carb diet can greatly inhibit the use of fatty acids (and the production of ketone bodies) via the presence of hyperinsulinemia.
