sanity check - constant running, loads and more

[Jonathan]

- if a stepper can be used as a driver for a shaft at low speeds and low loads

Yes, so long as the loads are actually low as steppers do not output much torque for their size.

- how one calculates needed torque for a stepper, knowing what the downward pull / load would be

The formula, which can be derived from considering the ballscrew 'unwrapped' to become an inclined plane is:
T=(F*L*e)/(2*pi)
Where T is the required torque, in Nm, L is lead (or pitch of the screw) in meters and e is the efficiency which I think is about 0.3 for a threaded rod leadscrew drive.