PWM = 50% sound about right to me. Shouldn't damage anything the voltage is low enough the existing insulation should handle it without even noticing.
Half bridge, definitely freewheeling diodes, and make sure they're properly rated.
1khz sounds fine, although you should tune the PWM frequency to a point where the motor doesn't produce noise under load. PWM'ing a motor will cause some kind of buzz, ever motor is gonna have a different acoustical resonating frequency, so play around with it. I'm gonna say you could go as low as 100hz and as high as the motor coils inductance will allow.