In May, an American investor tried to sell top German economic officials on an audacious plan to buy a Russian undersea pipeline. Despite years of international friction over the pipeline, he proposed to eventually activate it and deliver natural gas to Germany. The investor, Stephen P. Lynch, had already made the pitch to the Trump administration, which he was betting would want U.S. control over a pivotal piece of energy infrastructure. Now the Germans wanted to hear for themselves about Mr. Lynch’s proposal to lead a takeover of the much-criticized pipeline on the floor of the Baltic Sea, called Nord...