Follow-up to the recent follow-up: After casting the hearth and grinding out the flaws, and then leveling the depressions with cement slurry, I was unsatisfied with the resulting variation in color and texture:
So my mother and I built a new form around the existing block:
and poured (this time right-side-up) and troweled some new mortar (one batch for consistent color). I tried really hard to get it reasonably smooth and pit-free, but tolerated some bumps (which i could later sand down).
And then put some ledger tile on it. I actually just used construction adhesive, since i had some laying around and the area wasn't that big. The rock snippets came already glued into larger panels, i just had to cut them to shape. The wife kindly got some prefab corners which made it a lot easier to look nice (although the corner pieces are pretty tricky to cut to size).
Pretty happy with the look here. A little concerned about overshadowing it with a hideous concrete slab. We'll see.