OG: Great explanation mate.
Regarding retrofits to early designs, am I right in thinking that their wings were typically more highly outboard loaded than current practice (to minimise wave drag with their early transonic aerofoils)? So these wings have stronger tip vortices to work with and lift distributions that start closer to the non-planar ideal.
A question remains: Once you realise you have your 6% strength margin to spend on a wingtip modification, what are the considerations that would lead you to add a winglet rather than merely extending the span?