You're right about never knowing for sure, but many people in the know, like Churchill, thought Hitler's hash was settled after he declared war on the U.S. It was like an excellent heavyweight boxer deciding to fight three equally tough fighters at the same time. The final result was inevitable.
The only thing that might have saved Hitler would maybe have been if he had a sufficient supply and much more powerful V2 rockets. The Allies had nothing like that. Even Germany's use of jet planes wouldn't have been a deciding factor as the Allies were pushing them into production during the late stages of the war.
Hitler just took on too much. He underestimated the production capabilities, the manpower, and the determination of the Allies to see him dead.
Wrong. The V-2 was a total waste of time and money as a weapon of war. For the cost of the V-2 program, Hitler could have built a huge fleet of heavy bombers or Tiger tanks. Even if he had had 10000 V-2's it would not have made any difference at all on the outcome of the war.