Wiring a Battleswtich

[overkill]

0v across the motor, 15.4v across the switch which doesn't make sense? That's when it's being fired and when it's not

That is exactly what I would expect if the switch was wired correctly and in the off state. Just to be sure, does the motor run if you bypass the switch output with a length of wire? If it does, the power wiring is definitely OK.

The Turnigy switch instructions mention that it turns on when the radio channel its connected to passes 50%. Can you plug a servo into that channel and see it move fully in both directions?