There's no need to layer substrates. Using Black Diamond on top of anything else will result in the Black Diamond eventually settling to the bottom. If you're using an inert substrate, my advise is to pick one and stick with it.
If your substrate is inert - sand, Flourite, gravel, Black Diamond - you'll need to use root tabs and/or dose ferts.
If you go with something like ADA Aquasoil, you won't need root tabs. Depending upon your plants and specific tank, you may not have to dose any ferts for quite some time.
You can use egg grate, sure. Make sure the wood is water logged or it won't sink. If it's not water logged, you'll want to attach it to a piece of slate or something to keep it weighted down. Hot glue isn't great for aquarium use. Stick with super glue gel and the like.
Black Diamond is coal slag, not nickel slag. Most of us use the 20-40 grade.