[Al Skierkiewicz]

math,
The rule specifies that the minimum length anywhere on the robot can be no less than 6". The 3-1/2" corner rule specifies that soft bumper parts only may extend beyond the corner of the robot, i.e. a vertical pool noodle wraped with the same continuos fabric as the rest of the bumper. I believe the intent is to have a substantial plywood backing of at least 6 inches in length. Since the plywood may not extend into the corner (see fig 8-2 & 8-3) a corner section as you describe would have to be 9-1/2".
The Q&A might be a better place for this question.