They probably would have moved operations to Canada if the Islands fell. But then of course the Germans would have 50 million potential hostages if Britain didn’t give up the fight.
There was zero chance the Royal Navy fleet would have been defeated, let alone surrendered to Germany. Plans existed to move the royal family, the entire government, navy and as much of the army as possible to Canada. Furthermore there was zero chance that a German invasion would have succeeded. Of course that was not widely known at the time. The fleet would have been scuttled before handing it to over to Hitler.
I don’t think the fate of the Royal Navy entered any Americans’ minds in terms of isolation before Dec. 7 of 1941.