If it is 1.3ohms per coil then i think using half coils probably wont help. I was thinking maybe if it was like 8-9 ohms then half coils could of made bigger difference.
Then imho try change the battery to a atx psu. The battery is ideal power source only on paper, in practice it has internal resistance, and the stepper puts it on short circuit. Also the resistance is just a simplistic review, for the custom motor try check the inductance, if its higher than normal value for this resistance, that could be a reason for the behaviour. If all else failed to sove the problem, maybe try changing the motor, in case you have some other motor to do this test with.
Then imho try change the battery to a atx psu. The battery is ideal power source only on paper, in practice it has internal resistance, and the stepper puts it on short circuit. Also the resistance is just a simplistic review, for the custom motor try check the inductance, if its higher than normal value for this resistance, that could be a reason for the behaviour. If all else failed to sove the problem, maybe try changing the motor, in case you have some other motor to do this test with.