Here is what I would do:
Make the sheath and glue up the welt. Don't punch the stitching holes yet. Clean up the edge and sand with a 220 grit belt.
Cut a strip of 22 gauge copper just a little wider than the sheath. Maybe 2mm wider. I would leave an extra bit of length on the ends for final shaping.
Braze a bunch of 1/2" long piece of 20 gauge copper wire every inch down the middle of the strip of 20 gauge copper. Once all the pins are on the copper strip, pickle and clean up. Trim the pins to the length of the welt width. It wouldn't hurt to take a pair of side cutters and gently put indentions along the sides of the pins to add grip when glued up. Starting at the top, drill .75mm holes in the welt strip where the pins will go into the welt. Once all are drilled and the copper strip fits well, remove it and put 1 hour epoxy in the tiny holes. Use a piece of wire to run epoxy in the holes. Carefully put a tiny bead of epoxy down the side of the sheath. Re-install the strip and watch for any squeeze out. If any squeezes out, remove it immediately! Use rubber bands or something to hold the copper in place for an hour while the resin sets up. When the epoxy is fully dry (overnight) burnish the edges of the copper over tight to the leather. Punch the stitching holes and sew up. If the stitching awl hits a pin, it will just deflect a bit and go on through.