No, I do not have the full dossier although my source has (who kindly gave me permission to post the cover letter). Reportedly it is not that exciting and contains mostly information known from elsewhere, including Rutherford's propagandastic literature (such as Fascism and Freedom and Rutherford Uncovers the Fifth Column).
I agree with Octarine Prince and others that such a document must be viewed within its times, when people like MLK, Ernest Hemmingway, Frank Sinatra, etc. were also subject to investigation by Hoover's FBI. Similarly, the request is for custodial detention which recalls the injustices of the internment of American citizens of Japanese ancestry during the war. But at the same time, it must be put in context with Rutherford's own actions and activities. To think the matter was simply about "neutrality" or pacificism is just historical ignorance; Rutherford was anything but neutral. He was obsessed with "fifth column" rumor and constantly wrote and talked about how the United States government was being destroyed by totalitarian elements of society. His writings quoted from obscure anarchist publications and sometimes had "inside information" about upcoming plots (such as the "secret communications" that Rutherford quoted in his 1933 booklet The Crisis, which gave him advance notice of the Business Plot). He also got himself mixed up in the Bonus Army affair in Washington D.C., as Jim Penton chronicled in his recent book. His literature was determined to contain hate propaganda in various courts and he was commonly viewed as an agitator, especially with respect to the Catholic community who -- on account of his constant screeds against Catholics -- insisted that Rutherford be taken off the airwaves. The Ku Klux Klan approved of Rutherford's anti-Catholic broadcasts and the Golden Age accepted and published their supportive comments; the same publication later claimed that a Klan copycat group called the Black Legion were actually "agents provocateurs" trying to stir up agitation. Rutherford butted heads on the radio with fascist Catholic priest (and radio personality) Father Charles Coughlin, and violence sometimes erupted between angry Coughlinites and JWs (such as the Madison Square Garden riot in 1938). Rutherford fought the attempts to take him off the airwaves by having his friend (?) the far right-wing and antisemitic Rep. Louis T. McFadden (the font of many conspiracy theories today, who also tried to impeach President Hoover and the Secretary of the Treasury) push a bill through Congress to "safeguard the right of the people to hear over the radio whatsoever they wish to hear". Rutherford had thousands of JWs petition and campaign for this congressional bill....not exactly an instance of "political neutrality". Unlike Watchtower leaders today, Rutherford was a prominent American figure with a lot of influence in his day. And the FBI would certainly not forget that he previously had been imprisoned for violating the Sedition Act (e.g. for telling conscripts in boot camps to disobey the orders of their commanding officers) in the previous war, and his conviction was dismissed on procedural grounds (as opposed to being exonerated). I think internment was a terrible policy and Rutherford wasn't at all a threat like some people feared, but it is understandable why the FBI would investigate him and conclude that he might stir up trouble during war.
