Is it legal to lift the spring off the trapping (snap) portion of the mousetrap and attach the extension bar directly to the spring?

No, lifting the spring off the snap portion of the mousetrap would be a violation of 3.c.line 2 by modifying the mousetrap so that it would not function as intended and attaching the extension bar directly to the spring would be a violation of 3.c.lines 4-6, which states that items, other than the extension bar attached to the snap portion, may not be added to the mousetrap.