Could you use the highpoly to create a lowpoly eg remove all mesh smoothing and remove extra supporting loops? that might be the quickest way to get a good lowpoly of a hardsurface model.
Have to say that lowpoly is looking pretty janky, which is why xnormal might not have liked your UV's. You seem to be having a big problem with overlapping rays. Retopo manually. Don't rush things and use 3d coat for retopo especially for hard surface, it can be fine for organic objects but this bad topology and bake…