I mean… I wouldn’t do this. But millions live on a similar diet every day (minus the multivitamin). Sugary cereal for breakfast with fruit flavored “juice”, a fried chicken sandwich for lunch (kudos for the chicken, but then its white bread, white flour, soybean oil, and flavored soybean oil) plus a soda, and finally, say, boxed mac and cheese for dinner with a canned margarita to take the edge off.

Day to day, you will adapt and how you feel on the diet will start to feel “normal”. But you will get fat, be at higher risk for any number of health issues in the long term, and will likely feel depressed. But relative to, like, starving to death, you’ll be pretty healthy.

My intuition says that I’d wind up having a headache for the rest of my life.

Kind of yes, kind of no.

Short term there is not a huge difference between getting sugar from complex carbs or simple carbs and most vitamins and micro nutrients will be OK with a few weeks of worse absorption and slowly lowering levels.

Medium term this would be bad, but so is the standard western diet. Carbs are not a great source for energy for a number of reasons but one of the key ones can be seen with vitamin C. Why do we not have functional pathways for making vitamin C? Our closest relatives do, the other great apes, and almost all other mammals do too. In fact as far as I am aware one of the only other mammals missing the ability to make vitamin C is the guinea pig which is especially ironic considering it was the aminal selected to understand scurvy, an extreme form of vitamin C deficiency.

We don’t need anywhere near the same level of vitamin C if we are not eating sugars, complex or simple. Eating a very very low carb diet, deep into the ketogenic end, reduces the need for vitamin C. Taking someone who has symptoms of scurvy and switching them to a carnivore diet seems to reverse the symptoms fairly promptly and plenty of people eat just meat for decades at a time without developing scurvy, so it seems safe enough.

So if you look at a diet made of highly processed high carb foods like the current standard American diet you would see a measurable but not extreme change in the short to medium term, but in the medium to long term it would get worse. If you compare to a more reasonable diet which doesn’t have huge amounts of processed foods or carbs in it then it would be a bigger difference.

No.

you need proteins. neither skittles nor the vast majority of multivitamins contain proteins. And in any case, the vast majority of “junkfood diets” suck for your health. Usually they’re a gimmick to teach you about caloric intake for weightloss (and maybe some chemistry.)