Lots of good points and they're accurate. Our support for Britain initially would most likely have been air, troop wise not much, we only had about 2000 or so in Iceland. But in 1940 the draft had been resumed in the US right at the same time Hitler planned to execute Operation Sealion, which was in September. Any total subjugation of Britain would have taken weeks or months, if at all giving us time to train troops and mobilize them.
It would have been difficult for us if Hitler did succeed in conquering Britain before we could prevent it, but I believe during that period we would have gradually built up our forces in Iceland and possibly sent troops to Ireland for plans to invade Britain. Either way, he would have faced us directly and economically the US was a giant compared to Germany, we would have beaten him with or without Britain.
If, as with the 34th, the fastest one took a year, no reason to believe it could have been done any quicker. And that year is to get them to American ports, not embarked, arrived, unloaded, unpacked, and set up. Probably another 2-3 months to get there and get combat ready, so for a NG division mobilizing around the time of Dunkirk, it would be August, 1941 before they could fight in Britain.
I'm not meaning to slight the Marines, but they had their hands full in the Pacific, and, of course, some of the Army divisions had to go out there too and couldn't have helped Britain.
No doubt in my mind that Churchill is The Man of the Century. Nobody else alive (and maybe nobody who ever lived) could have kept their people's heart in the war.
If you'll allow one more thought: I've become a really WWII nut in the last year. Someday the gang needs to have a WWII book review-discussion-round robin. I love Bookfinder.com - and they love me!