"From a historical perspective, does anyone know how the spanish heel design made its way into ukulele building? Is it a "traditional"
The first Ukulele makers inherited the construction techniques from the Portuguese school of guitar making, which in its turn was influenced by the renaissance-baroque guitar making tradition which already had 300 years of history before Nunes, with the fingerboard flush with the soundboard low action, gut strings, etc its easier to utilze a "spanish" neck when the soundboard and fingerboard are flush, far quicker, I am sure you have seen the 1919 video!!
you can also still see the influence of the original Spanish Vihuela/baroque guitar and Portuguese guitars all over South America, re-entrant tunings, courses (like taro-patch ukes) charangos, cuatros, tiples and more though these have evolved over hundreds of years to available local materials.