have the video preview keep up with the start and end points as I drag them from side to side with the mouse.

It is good that the space bar pauses the preview but after pausing you should be able to use the left and right arrow keys to move forward and backwards by one frame at a time. You should then be able to press the 'S' key to move the Start point to the current location, and the 'E' key to move the end point to the current location.