I can't select either of the above answers as the best answer for the following reason:
The brownie I'm making is meant to be "gooey", and even after cooling for 2 hours, and then chilling in the fridge for another 30 minutes, the the upper-half brownie still remained fudge-like. I ended up using Jay's suggestion to eye-ball a 4x4 grid, however I didn't use a hot knife. I used a plastic knife with an up-and-down sawing motion and got fantastic results. The cuts were as clean as you could ask for, and the entire process didn't take more than 5-10 minutes.
As an aside: I decided to forgo the "aluminum foil sling" and just spray the 9"x13" pan with PAM, line the bottom with parchment paper, and spray it down one more time. The brownie "loaf" came out with zero fuss and had a perfect looking crust.
edit: I plan on experimenting with 2 large metal rulers to score the loaf, instead of eyeballing the grid.
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.
Best Answer
Multi-layered brownies are certainly possible, however if you try and make it too thick it can be too dense at the bottom because the weight won't allow any sort of rise (most brownies do rise, just not much but it's still important). I make a triple chocolate brownie with a layer of salted caramel in between and it comes out really well, that's a single recipe split into halves.
Your idea is to make two boxes and then do 4 layers, it may work just fine, if it were me I'd do it in two batches and then place the top 2 layers on after they all cool.
As for what to put in between you'll want to choose ingredients that are markedly different from the brownie itself. Caramel contrasts as does white chocolate, but a layer of chocolate in chocolate brownie is less likely to be noticed.
As for the amounts of oils and eggs, follow the recipe to the letter, don't mess around with it as the consistency is already optimized. If you want to experiment do it in small batches so you can measure the results before committing to a big bake.