Which national engineering prowess largely drove the final commercial perfection of the derailleur post-WWII?

Italian engineering.

The technological progress in developing high-quality, mass-producible derailleur systems was significantly disrupted by World War II. Following the conflict, the final steps needed to create a truly practical, reliable, and commercially successful derailleur were largely spearheaded by Italian engineering expertise. This intensive effort culminated in Campagnolo introducing the Gran Sport model in 1949, which successfully integrated key features like the parallelogram design and reliable cable actuation, transforming the concept into the standard for high-performance cycling.