A former federal prosecutor has emerged as special counsel Robert Mueller’s most persistent courtroom critic. It’s not Rudy Giuliani, a former U.S. attorney and now President Trump’s ubiquitous defender, or any of cable TV’s prosecutors-turned-pundits. He is Eric A. Dubelier, a litigator for the Reed Smith law firm who knows international law and the D.C. playing field. He served eight years prosecuting cases as a Justice Department assistant U.S. attorney in Washington. He refers to his former employer as “the real Justice Department,” implying that Mr. Mueller’s team is something less. His biting remarks have come in months of court...