Ah you are taking about the second one? Sorry thought when you said single fly you were meaning the first one. Regards the long leader it can be three dries, so long as they are tapered in terms of wind resistance. Eg Sedge, Shipmans, Suspender. Which would be a standard set up for me. You are right; if it was replaced with a single fly, without tapering, then I’m sure it will collapse.
Cheers, Paul