If your camera shutter is set for slow shutter speed, you can press -- and immediately release -- the shutter button of the motor winder, and the winder winds on only after the completion of shutter close.
Yes, you can use the winder with mirror up. If you fire with mirror up, the winder winds on and for a brief moment you will be able to see thru the viewfinder, but then the mirror flips up again.
Yes, you can remove film back and set body for multiple exposure, and 'practice'