Had Coup Succeeded...
I think the attempt to kill Hitler was "too little, too late" to make much of a difference for Germany. The attempt was made in July of 1944, after D-Day and after Russia started closing in on Germany. Total surrender by Germany was not until April 1945, but the Allies were well on their way when Operation Valkyrie was executed.
I think had the plot worked, the best that Germany could have hoped for was an immediate unconditional surrender to the West while trying to keep the Soviets out of their country. Germany knew that the Soviets were going to want revenge after the horror they had inflicted in the early days of the war and, maybe, a surrender in July of 1944 could have stopped communist takeover of East Germany. I doubt it though- Stalin wanted blood and knew he would soon get it.
At best, many lives would have been spared but Germany would still have been taken over by the Allies.
An attempt in early 1943, however, might have made a real difference. If Germany agreed to withdraw all of its forces out of occupied Europe then I think they might have been able to avoid being conquered.