I can tell you have a lot of creativity and natural ability to solve problems. Hard to tell yet if it's taking you in a positive direction.
That said, did you ever consider actually using the AH slates on the GC? IOW, were they close in size?
During the 80's i built tools including drill guides for slate, for a small scale custom table builder, but never worked on one myself. As i've said in the past, wish i had paid attention, but my attitude was "John will always be around, why would i have to learn that. " :sorry:
Where I'm going with this, i have been to the same quarry a number of times since, and bought slates for architectural uses including sinks and counters. The quarry makes impeccable joints, but where i make custom cuts or didn't specify a close size, i can match them with a diamond blade on a skilsaw. I butt the slates snug, and clamp them down so they are level, shimmed if necessary, to flat wood ledgers underneath.
Then clamp a straight batten or aluminum level that extends at least 6" wider than the cut, both ways, and run the skilsaw down the joint with water playing on it & right behind the blade. Obviously there is a bit of set and try to get the batten aligned so the blade will be in the existing groove. Slate can be cut dry, but trust me, you don't want to. Certainly not indoors. It will also spall more if cut dry, because it heats and pops.
After that, slate can be filed with coarse files, or with a straight batten with sticky-back coarse grit sanding discs wrapped on an edge.
I have no idea about Brunstone.
smt