Bottom bracket compatibility
You cannot use a 'threadless' bottom bracket in a threaded shell. Perhaps you are thinking of two-piece bottom brackets designed for press-fit frames that thread together and are designed to eliminate bearing misalignment and creak. These only work because they fit very accurately into the press-fit bearing cups.
The 127[.5] measurement refers to axle length. Each crank model requires a different axle length to get the correct chainline. 0.5mm will not make a difference.
What I'd do is have the BB shell faces faced to give a clean flat surface to the flanges of BB to butt up against, and clean the threads as much as you can will a wire brush and pick. Install the BB with a generous amount of thread locker.