If you only want three speeds, off, half, full, off and full are easy and don't require any work. For half, I think you're over-engineering your options here. Why not just do the wiring such that you can have the pack in full series, or half turned off? A simple switch that moves a wire from touching a lead to the motor in the half position, can then switch it over to touch the next battery terminal in the full position. It will be a little jerky, but you're only doing this once a flight, so it seems like that wouldn't be a problem. I think a transformer or resistor is too much work/a waste of energy. Just installing a lever that will pull a lead from the rest of the pack to a motor terminal should work fine.
But I have absolutely no credentials, not even EE for dummies, so take that with a grain of salt.