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 recipe you've found looks like "crazy cake" or "wacky cake" (or "depression cake"), which is a fairly common egg-free cake. I've made it a number of times for a friend with an egg allergy and it's a very good vegan cake.
To make a vanilla version, leave out the cocoa powder and increase the vanilla extract slightly (1.5 teaspoons). There are a wide array of variations on this blog -- chocolate is the "original" (and most common version that I have seen), but vanilla, spice, lemon, and more all seem to be possibilities.
Best Answer
The primary difference between cream, condensed milk*, and regular milk is fat content. Cream has the most fat (about 36% fat), then condensed milk (about 9% fat), and finally, regular whole milk (about 3% fat). A cup of cream has about 12 times more fat than whole milk!
Increasing the fat percentage in your cookie dough will lead to a softer cookie. I imagine cookies made with cream would be especially spread out and soft, whereas cookies made with condensed or evaporated milk would be somewhere in the middle in terms of softness.
Substituting these ingredients may be useful if your cookies are turning out harder than you want them to be. However, it's also possible that these modified cookies may not hold together as well as the original recipe.
*As mentioned, using sweetened condensed milk instead of condensed or evaporated milk would require you to dial back on your sugar content, otherwise you will get a much sweeter cookie.