SaltyShrimp makes a KH only remineralizer. You might be able to use that to buffer your water so you don't have to depend on a substrate. I'm thinking about that myself so I can use inert but still maintain an RO/tap mix.
It shouldn't be inert. It should alter params just a little.
Check with Frank when you place your order. This is what the ADG website says about Ryuo stone:
"This decorative stone type replicates the look and feel of a Japanese Zen Rock Garden and lends itself well to the Iwagumi aesthetic for design in an aquascape. These stones are inert and will not raise water hardness or ph values."
I only know because I have really insanely hard tap water, and ordered the Ryuo stone so that I wouldn't make things even worse by using stone that raised the pH.
As far as shaping the stones using the patio, it came out just fine for what I wanted. However the fractured edges will be much darker than the weathered surface of the stone. I think if you've done lots of visualizing and know exactly what you want, Frank should be able to find you just the right stones. Enjoy your planning!