I'm a student of WWII and I've never heard anything like this before. FDR was helping out England long before Pearl Harbor, and against the wishes of Congress and possibly the majority of American people, who wanted to remain out of the war. FDR came up with the idea of Lend/Lease which allowed him to send weapons and supplies to England and later Russia, even though we were not yet in the war. I doubt that there was ever much of a chance of the US joining with Hitler. The Jewish owned banks would not have allowed it, for one thing. And let's not forget, there were Americans fighting WITH the British well before Pearl Harbor, notably in the Eagle Squadron. In fact, there was an American "observer" on Catalina Z of Coastal Command, which located the Bismarck after she'd slipped away from the British cruisers which had been tailing her. ("Pursuit" by Ludovic Kennedy)
As for the Empire being an obstacle to trade, I must admit I'm not very well informed on economic activities of the era, so I can't really comment on that. But this is definitely the first time I've ever heard anything which suggested the possibility of the US joining Germany.