Magnesium nitrate is unnecessary nitrogen and probably magnesium. Most forms of magnesium are soluble, so if you actually need magnesium (in soil you probably don’t), use a sulfate. There are not very many soluble sources of calcium, so if you need nitrogen, you can always use more calcium!
What foliar questions do you have? Maybe we need a specific foliar feed thread in the nutes subforum.
Composting all those amendments would t change the mineral content of them; only how available they are. Leaching (runoff) would be the only way to reduce some of those.
I would not add all those amendments before testing because once you add them; you can’t get them back out and you may and severe excesses. That’s like baking too much salt into a loaf of bread, good luck getting it back out! I would mix all your base soils you have laying around, and mix it as well as you can, and then take a soil test and then use the amendments that you actually need.