I know this is a dumb question but how does a cunningham control the leech? I thought it only controlled the luff. Maybe you’re mixing your luffs and leeches. With my sails I obviously fitted a bolt rope I didn’t bother with a leech line.
I aggree that the sliding gooseneck tends to drift up the mast. When I use a fixed gooseneck I haul the sail so that the boom is say 75mm above the fitting. I then cleat the halyard and pull the boom down to the gooseneck whick stretches the luff nicely. Robert