With all major participants involved by Dec 1941 what would the be the Earliest possible ending to WWII in Europe if the Allies had played their cards absolutely right. Similarly, if the Germans had played a blinder, how long would the war have gone on for. It think it is safe to say that it was impossible for the Germans to win at this date - the combined opposition was so strong. The judgement relies on the opposing side playing as well as they did in the OTL.
My guess is Nov 1944 would be about the earliest end. D Day could not have been earlier than June 1944. Anglo American airforces could have knocked out the oil supply and 'Market Garden' should not have been carried out on intelligence sources.
The other question is a lot harder and I dont think I have enough knowledge to give my guess on this.
I am guessing this one has been done before but I could not find it using the search facilities, so apologies if that's the case.