From what I can tell and what I've read, you'll get better results by mirroring horizontally. I don't think you should attempt to mirror both horizontally and vertically at the same time like in your latest examples because vertically just does not work well at all. If you look at your new examples the horizontal mirroring…
@Eld Thanks a bunch for the response. Yes, in the above example I tried both those tests with another model that had the geometry completely split (the polygons were seprate), and another test where the verts were merged but the smoothing groups were seprate (hard edges), and a final test where the verts were merged and…