A couple of opinions:
If Von Stauffenburg had just thought to have placed the unarmed 2nd charge in the briefcase with the armed 1st charge, it would have detonated and surely killed Hitler and probably every other officer in the room.
If Hitler HAD been killed on 7-20-44, I think the new German Commanders would have sued for peace immediately. They knew it was all over. They would most likely have had to go ahead and settle for unconditional surrender, it may have taken a couple of months to effect, but think of the millions of lives and destruction that would have been spared.