Interesting idea. I've done this race 3 times since it first started and never heard about any such restriction (but don't take that as being a green light to do this). I guess the alternative route you describe is a slight variation of the Four Inns course, but no need to visit Hey Edge CP, so can take a more direct line down to Crowden (as per Holme Moss race). I've always gone via Sliddens.... but not for any particular reason. Using Tracklogs, appears Sliddens is 0.2 mls shorter, but with next to no difference in ascent (no major surprise there I suppose). Can be quite boggy running over Westend Moss. There is a reasonably hard track to be followed after Crowden Little Brook if you go via Sliddens, which could prove a tad quicker.