I have two questions.
First, during the qualifying portion of the competition (when you place your robot in the measuring box to ensure it meets the criteria), are you allowed to have your robot on? Can you use a motor to pull an arm down in order to meet the 18" guidelines?
No, your robot must be in the 18" x 18" x 18" with no power during inspection (qualifying in your terms). The robot when placed on the field at the start of a match must be as it was inspected. No power is available.
Second, if you are using rubber bands to create tension, can you have them “hanging” on only one part of the robot during qualifying, i.e. measuring, and then attach them to both ends of the robot to create the needed tension during the match?
The answer to this is yes, but only if attaching the rubber bands does not cause the robot to be outside the size limitation.