Sometimes it is the quiet, elusive ones who come back to haunt you. And for ex-special prosecutor Robert Mueller, one of those might be a Russian billionaire named Oleg Deripaska. The oligarch who once controlled Russia’s largest aluminum empire has been an international man of intrigue in the now-completed and disproven Trump collusion investigation. Deripaska was a disaffected former business client of Donald Trump’s fallen campaign chairman Paul Manafort. He also was a legal research client of Trump-hating, Clinton-aiding British spy Christopher Steele. In his spare time, he was an occasional friendly cooperator with the FBI and its fired deputy...