So to at least address some of your Qs as I understand them:
Metallic, yeah, we all tend to use maps for metal (I tend to just reuse the R/G/B channels from the BaseColor texture to save on samples). Otherwise the PBR model tends to assume Metal is an integer for a given material. I think it’s just a holdover from olde times when that was the case vs just-plugging-a-texture-in. In that sense 1 would just be a white field, so not worth binning into the VT.
Opacity controls something about how strong the texture blends/renders. I never use it so I can’t comment from experience.
As for the flickering, it looks like Z-fighting? Did you set your source-landscape to Never render in the main pass? You don’t need it as it renders to the RVT…
Otherwise, from my experience, I can say that when I adjust my snow-value (for example) and it would otherwise smoothly fade to white, the RVT has to re-render all the tiles, and I could see over ~1 second all the tiles update 1-by-1. I came to the conclusion real-time adjustments and the like (including depth-fades, etc) aren’t viable for areas where you can see many many tiles…