Complex sentence: Testing and evaluation revealed several innovations, including an all-metal hull and, when compared to German flying boats that were also undergoing evaluation, simplified construction, features which the Japanese Navy hoped to incorporate in a replacement for the Type 15 Flying-boat.
Split into simpler sentences: Testing and evaluation revealed several innovations when compared to the German flying boats that were also undergoing evaluation. Those features included an all-metal hull and simplified construction, both of which the Japanese Navy hoped to incorporate into the replacement for the outdated Type 15 Flying-boat.