if coco is buffered properly it does not cause any change in nutrient requirements relative to any plant you put into it. When properly buffered it will not leach or release nutes as far as what is available to th eplant. coco is a pita, i'd just go with typical soil mix and add 'enough' drainage amendments to get to a 50/50 gas:water mixture, which is the goal.
sure you can mix it in to the soil. If you are making a soil, i don't see a need to use it unless it saves some money etc. Better to keep it simple and just mix your soil any nutritional amendments with equal parts perite or vermiculite #3 or similar. The correct gas:water ratio does not require 3 or 4 different media. Playing mad scientist may make you feel like you are doing something unique and helpful, but it really just adds a bunch of effort for zero benefit. You can use one and add the draiiange elements as needed. If you use coco as the base, you only need 33% perlite (2:1) or similar amendment. consider the ratio of drainage element when working out how much nutrients to amend into it. if you had 1:1, you've basically cut the nutrition per volume in half.
If you do choose to mix multiple media, do a weighted average for the necessary perlite or similar to add.