Pure speculation indeed but we can certainly quibble.

Roosevelt was Machiavellian when seen from the perspective of the Commonwealth - the US entered the war where it was convenient for the US, not where it was convenient for the Allies. And Stalin's brilliance was Soviet infighting, not strategy or politics; nearly everyone of consequence had supported the White Army to some degree and Soviet warfighting had pretty much revealed itself to be an appalling appetite for attrition. "Listening" to the Poles? Not enough to do anything about it.

It's entirely possible the Russians would have continued into Berlin. If so, they would have done so with all the materiel the Allies didn't land at Normandy; a failed invasion in France would probably have resulted in a redoubled effort in Italy.

The Nazis made several overtures for a separate peace with the Americans. They were rebuffed because the Americans wanted unconditional surrender. A failed invasion at Normandy would have changed an awful lot; Hitler would have been reenergized, the Russians would have bogged down, and we might very well have ended up with a really ugly, heinous Europe. It would have collapsed pretty quickly anyway as German industry was effectively destroyed by then. I think we would have seen the Balkanization of Nazi Germany a good 40 years before the Balkanization of the Soviet Union.