@StainlessSteelRat linked to a very good resource for stepper motors at All About Circuits, but I fear he didn't address your questions. I'll go through your question line by line.
I'll probably have a little spool out there to hold the string for the weight so that probably changes how much torque it can deliver too.
First, the sentence above is wrong. The radius of the spool will not change the torque. It will change the weight you can lift, but only because $Fr=T$, where $F$ is the force available for lifting, $T$ is the torque output, and $r$ is the radius of the spool. BTW, this method works fairly well, I have done it myself.
Anyway all I see in this datasheet is "in traction torque", and I'm not sure what that means
Based on the torque curve from All About Circuits, it is probably almost equal to the holding torque. As you see from that curve, torque is fairly constant at the low end of motor speed. All About Circuits also mentions that in stepper applications, the speed of the stepper motor should be gradually accelerated.
What happens if I run it at 5V instead of 12V for instance. Or if I current limited it to 1mA.
If you run stepper motors at a higher voltage (within rated limits), the current, and therefore holding torque, will go up. Another way to look at it is that you can run the motor faster for the same torque. I don't have any equations here, but increased current leads to increased force in electromagnets. If you current limit the solenoid, the holding torque will go down.
One last note: @am304 makes an important note about half/quarter stepping. The torque will reduce for half/quarter stepping because the magnets in the stepper motor are acting in opposing directions, reducing the net torque.
Sorry I don't have any equations, but experiments are fun right?