The excess voltage was almost guaranteed to be a major contributor to the problem.

You still need to verify that there is no excess binding in ghe drive line. a tiny warp in the shaft can cause a lot of binding n the tube. A minor misalignment of the motor to the drive saft can also cause binding. Any binding will cause the motor to overheat. An ovesize prop for the motor would also cause excess load... and overheat the motor.

Inadequate cooling would almost never cause the insulation melting on the leads between the motor and battery. You've got to be putting too many amps through th motor for that. You might check what the max amp ratiing for the motor is and put a fuse in the + lead. Then if the prop is oversize and/or there's drag in the drive line... the fuse blows rather than trashing the motor.