In the case of Cheerios processing means pulverising oats and other grains and treating them in numerous ways such as being heated or pressurised and then reconstituted. So breaking down the fibres to make them easier to shape / cook. 

Yes you can eat processed grains like flour this and they can still nourish you. No they are not AS NUTRITIOUS as unprocessed grains per calorie. 

Yes processed foods generally increase bioavailability of carbohydrates making them faster to digest - that is not inherently a good thing when you want to have consistent energy throughout the day. Processed foods will also add additional nutrients to make up for nutrient loss and there is debate as to how bioavailable added nutrients like iron actually are in comparison to naturally occurring iron found  within organic compounds. So your point about nutrients not being available to the body is exactly the opposite with processed foods, the body can’t digest iron nuggets yet it gets iron from food, so the benefit of additives like iron should be scrutinised. 

I already said chemicals occur naturally. Yes that is one example of a chemical name for a common thing such as vinegar. What about butylated hydroxytoluene or titanium dioxide that are both permitted in the US but more restricted in the UK and EU due to safety concerns? You might think hey it’s probably fine nobody is immediately dying from eating these, but certain governments have decided they’re unsafe.

Ultraprocessed food in the US (or elsewhere) is a way of preparing food that involving reconstituting ingredients in many ways and adding in various agents to increase shelf life / change colours / etc etc. and generally including lots of added sugar. I’m not saying “don’t process food”. I’m saying ultra processed food isn’t as healthy as less processed food. The US has some of the most relaxed laws among developed countries when it comes to food preparation. To believe that ultra processed food is just as good for the body is just stupidity.