Correct. The fat/carb/protein ratio is 47/33/20 for 2.0 (liquid version) and 45/40/15 for 1.5 (powder version).
Some of the things you listed are for macronutrients and some are for satiety or possibly to improve emulsification.
They include the essential vitamins and minerals that are needed for health. (As identified by the FDA.) These are needed for body function. Soylent is meant to be a complete source of nutrients, and so, by definition, these need to be included.
The chemical names being listed are likely due to reporting requirements imposed by the FDA, but it could just be a listing preference. I am not sure why that matters.
Like what? Name them.
Everything that has been proven necessary so far is indeed "in there".
There are some things (like phytochemicals) that science does not yet know if they are needed in a diet. The great thing about Soylent is that if anything is discovered to be necessary, it can easily be added in the future.
There is fiber. It is listed right on the nutrition label.
It is actually quite reasonably priced compared to the typical American diet and they just recently underwent a price cut.
"They" (I am assuming referring to people in general) can do anything they like. There is a vibrant community of people which do indeed "mix it up themselves" by way of DIY soylent.
Carnation Instant Breakfast would be a very poor nutritional substitute for Soylent in general, but people are free to consume whatever they like.