Backstroke Rules - Some Questions

Hey Fellow Swimmers, I am a backstroker and I have a fairly good underwater SDK for my age. I can swim faster underwater than I can on the surface. Usually, 15 meters is not a problem for me. From the USMS rule book: 101.4 Backstroke 101.4.2 Stroke—Standing in or on the gutter, placing the toes above the lip of the gutter or bending the toes over the lip of the gutter immediately after the start is not permitted. The swimmer shall push off on the back and continue swimming on the back throughout the race. Some part of the swimmer must break the surface of the water throughout the race, except it shall be permissible for the swimmer to be completely submerged during the turn, at the finish and for a distance of not more than 15 meters (16.4 yards) after the start and each turn. By that point, the head must have broken the surface of the water. My main question: Upon reviewing the rules, I believe that I could break the surface of the water with my head before the 15 meter (16.4 yards) and then continue to swim underwater (SDK on my back) for the remaining part of the race as long as I held my pinkie above the water (or until I drown). A minor question: most pools have lane markers embedded in the lane lines - how does one know if the markers are exactly 15 meters (16.4 yards). It does not appear that there is a stringent requirement to measure the markers (as opposed to the length of the pool). A minor question: does the 15 meter rule hold for 25 yard competition. I was told that for 25 yard competition, the maximum allowable distance is 15 yards. Respectfully yours,