driver side brakes heating up way more than pass side

Your next test would be to drive with the brakes disconnected AND the break-a-way switch disconnected and see if the hubs get hot. That test will determine what to look at next.

Having slightly different voltages is normal side to side with standard trailer wiring. If your brake wires go to the drivers side first then cross over to the passenger side then the passenger side will have a slightly lower voltage at the magnet due to higher resistance from the longer wire run.