The differing tensions are a result of the dish. Equal length spokes at equal tensions on both sides of the hub will center the rim between the hub flanges, and it's how I built my rim brake front wheels. Doing that on a rear wheel with unequal flange spacing from the center will still center the rim between the flanges, which is not the center of the hub.
For clarity's sake, don't continue to refer to the sides of the hub as "DS" and "NDS" simply because the freehub is on one side and the disc mount is on the other. Your DS will always be the driven side, and the NDS will always be the side that is there to support the rest of the rim. You'll confuse and irritate poor Bill, and he'll ignore your future cries for help. You should come back later and let us know how you like a fixed MTB wheel.