It depends a lot on what you expect. I wouldn't call it a mess. The result will be edible, and will have a similar flavor to a brownie. The texture will be very different from a brownie. It will be dry and dense. Personally, I wouldn't eat it.
First, the fat plays important roles in baked goods besides taste. I am surprised to see a recipe which uses melted fat (so no creaming) and no baking powder. This thing (the original one) is going to be denser than chewy cookies - maybe OK for you. If you reduce the fat from the original recipe, you will get a very dry result. An airy cake which is dry is not too good, but can be eaten when combined with e.g. yogurt or other wet things. Something which is both dense and dry will taste like chicken food mix run through a dehydrator. If you have a recipe for baking, don't reduce the fat. Also, never use spreads or margarines or anything else which is not real fat. It does not act like real fat, so it doesn't work the way it should. It is likely to contain lots of water and gums, and melt into them when baked. For baking, always use butter, lard or shortening. (It is OK to use vegetable oils for dense recipes which direct you to melt the butter). You will need your full share of fat. Some people replace part of the butter with applesauce, but you can't replace all of it. Also, I don't know how it will act in a non-leavened recipe.
Second, you need the sugar the same way you need the fat. It retains moisture and makes the brownies soft. Honey is already a risky proposition, but partial replacement will work. But do not reduce the sugar by half, it will make dry, unpleasant brownies. Also, I don't know what "no calorie sugar" is, but it sounds like some artificial sweetener dispersed in a filler. I don't know what the filler is made from, but it may not be good for baking.
You can't always replace wheat flour with other flours, but finelly milled oats should work for brownies and cakes.
Using a vegetable or fruit puree in place of the liquid is normally a good substitution, but your recipe doesn't have liquid. Just adding cabbage will destroy the liquid balance. You could theoretically replace part of the eggs with an equal amount of cabbage puree by weight, but because eggs also have important roles in baking (they lubricate and set the dough), you shouldn't nomrally replace them.
So, the replacement will bake into something you can bite and chew and swallow, but I wouldn't call it brownie. It also wouldn't be a tasty non-brownie, at least not in my eyes. Also, if you go ahead and make the stuff, you don't even win much in calories. Your original recipe is about 2950 kcal, and you get 820 g of brownies, resulting in 359 kcal/100 g. Your replacement recipe is 1610 kcal but only 530 g of brownies, which makes it 302 kcal/100 g (still a very caloriedense food) - and that if you use these substitution products, which are bad for baking. So, you only lose 15% of the calories overall, but you get something which tastes nowhere near the original. It will have cocoa flavor, but it won't be moist and creamy. I would rather eat 100 g of real brownies than 115 g of brownies made with your recipe.
The answer is mostly just "it's tradition", as with most questions like this.
I do think the pattern you've described isn't quite the actual one. What really happens is that we tend to eat peanut butter and jelly on sandwiches, and put one or the other on single slices of bread, because with a sandwich you can spread one thing on each half and put them together, but with a single slice of bread it's messy. Sandwiches might be on toasted bread, though - untoasted is common and easier but plenty of people like toasted bread for sandwiches. Similarly, there are certainly lazy people out there who'll just spread something on a slice of untoasted bread.
Beyond that, it's just what we do.
Best Answer
I like peanut butter, but the thing with using neat peanut butter in recipes is that it can be very cloying and weld your mouth shut. Mixing peanut butter with butter helps loosen it up a bit, the sugar is there to add flavor back and keep it sweet as butter is a savory, and not a sweet flavor. Peanut butter will roll in balls just fine without any extras, they are just there to improve the eating experience.
Having a big ball of peanut butter in the center of a cupcake is problematic because cupcake batter is fairly slack, however brownie batter is thicker so I it's less likely to run out the bottom.
Freezing the peanut butter is a good thought, however it is likely to keep the center from cooking - the outside of the brownie will be done but the center will be very underdone which is probably not what you are looking for. I'd suggest trying the balls at refrigerator temperature first.