2)Since they are 6 wire motors, the current spec would be for the unipolar current. Half winding bipolar would use the same current as unipolar so 6 amps should be the max for half winding. For full winding(ie bipolar series), the the current should be even less, a maximum of .707 times the unipolar current or about 4.25 amps. I hope you haven't damaged the motors.
1) Unmodified, the best for a 6-wire is usually half winding. The advantage to converting to 8 wire is that you can run bipolar parallel which will allow higher currents and consequently higher torque. For most motors, the max for bipolar parallel would be 1.414 times the unipolar current. Be very careful if you decide to rewire! You should not remove the rotor from the motor. Mine had just a cap over the wiring connections and removing it left all the bearings in place, so they were easy to rewire. On some motors the cap covering the wiring is also the bearing housing, so you have to be careful with the rotor or you can demagnetize it.
3) Gearing up or using a coarser pitch screw would reduce the motor speed, but would also increase the torque required. Any performance difference would depend your machine and the torque curves of your motors.
Don't know about #4