Question on cold setting the MTB frame - why not just add spacers to the hub?
I've cold set a road frame from 126 to 130mm.
I also bought a vintage rear hub to keep the spacing at 124mm (I think, it seemed odd, it wasn't 120mm or 126. The frame is a mid-70s Schwinn Super Le Tour 12.2.).
I'm a fan of coaster brake single speeding for rainy season dirt road riding. My coaster brake wheel needs a re-build. Keeps breaking spokes.