Your making this more complicated than it needs to be. Use a 2" base of miracle grow and put a 1" Black Diamond cap on it and your done.
Messy plant removal is more about you than it is about the dirt. The trick is to turn off all water movement devices, then slowy wiggle the plant out holding it at it's base. Some people only pull 2-3" of root up then snip the roots of at the substrate leaving them in the substrate to decompose.
You still may get a little dirt out but that can be easily syphoned off.
There many ways to do slopes, eventually they all tend to level out. If you do try to slope the substrate keep it moderate.