I think your oak leaves look great! Great work.
For the stitches I would get a seam ripper to cut your thread to take apart & re-stitch. You probably will have to cut every stitch with the seam ripper.
I'm just learning back grounding myself, but I think the checkered back grounders are easier to use. I got a pebble back ground, that seems harder to execute.
I'm not sure on measuring for your welt, I always lay the knife in the sheath, before it sewn & visualize my welt shape.
Also to make your sheath look better/more professional, you should burnish your edges.
Nice work & thanks for sharing.