re: Would the Allies have won WWII without America getting involved?

Another thing to consider is the division of German forces. Troop placements in France and Italy to forestall American-British invasion/advancements as well as the considerable Luftwaffe presence throughout Europe could, for the most part, have been redirected to the easter front. Britain alone in the west would have lacked the capabilities to engage in any sort of significant offensive (with or without lend-lease). The article linked indicates that the Luftwaffe was primarily tied up in the west. The addition of the German air force alone to the eastern front would have been significant.