Agreed. Should be the same positive or negative.
The tail tends to have a lot of resistance on these types of lights. You could copper braid the springs. All the batteries connect to the switch and then to the trace that goes to the edge of tail board. You could braid from the switch to a copper or brass washer where that one mounting screw that touches the switch trace. Also, you could make sure there is good clean contact between the tail board and the body at the 4 mounting screw points.
With that charging port, you could use texaspyro's driver (when it finally comes out) and put an electronic switch there.
If you have the driver space and decide to go 2S or 3S, you could fab your own driver contact board and use this driver: