The 20th July plot was doomed to failure from the start. The conspirators can be considered at best amateurish and at worst even more incompetent than Hitler himself. This guys wasted precious hours after the bomb exploded at Rastenburg wandering if Hitler was dead and waiting the arrival of von Stauffenberg to hear from him what had happened, without doing anything significant in the mean time. When they decided to act, the whole plan was extremely lacking. For example, there were no attempts to arrest the most important National Socialist leaders, like Himmler, Göring, Goebbels etc. There were no attempts to send forces to occupy the Wolf's Lair and arrest all those present. There were no attempts to take control of the OKH and the OKW. Generalfeldmarschall von Witzleben, who was to be the new Head of State, didn't show up until 8 p.m., when it was clear that the plot had failed. This guy was supposed to address the German nation a message about how the SS killed Hitler and launched a failed coup and how he and his fellows crushed the rebellion and assumed control of the state, but he was too lazy to write the message, so no broadcast was made. And this were insignificant blunders, when compared with the following two failures. General Fellgiebel, who was in charge of all the communications at the Wolf's Lair, proved unable to cut Hitler's private lines, so he was able to direct the crushing of the plot. The other major failure was that the conspirators ordered that the Propaganda Ministry be surrounded by troops, but didn't order the cutting of the communications, so Goebbels was able to call at Rastenburg and obtain a direct line with Hitler. Then, he asked the commander of the forces around his Ministry to come and speak with him (Goebbels). When the officer arrived, Goebbels asked him if he was a National Socialist. The answer was affirmative. After this, Goebbels gave his phone and invited him to speak with Hitler, which gave him orders to crush the coup. And this were just a few examples of incompetence off the top of my head.
But let's assume that the leaders of the plot were at least somewhat competent, that Hitler was dead and the conspirators were able to take control of the state. Would they have been able to reach an agreement with the Allies? To be able to answer this question, we must look at the ideas of one of the leading figures: Carl Goerdeler, the new Chancellor. The most striking feature of him was that he greatly surpassed Hitler's wishful thinking. Just an illustrative example: in 1943, he expressed his certainty that, if he met Hitler, he would be able to persuade him to step down and let him (Goerdeler) to assume control (I am not joking, he really thought this). He also thought that Germany was entitled to annex parts of Poland, Alsace-Lorraine and other territories. His offer for peace was something along the lines: Germany retains its gains prior to the start of the war, the annexed Polish areas (including those with Polish majority), Alsace-Lorraine, South Tirol and a more advantageous border with the USSR (read as annexing territories with non-German majority, because Lebensraum). This were his conditions, despite Germany was losing the war on three fronts, with no hope to achieve a favorable stalemate. Do you think that the Allies would have accepted this delusional offer?
My opinion is that, even if the coup would have been successful, the fate of Germany was sealed. The only positive outcome for the German people would have been that the plotters, being less fanatical than Hitler, would have capitulated earlier. We must not forget that the 20th July plotters were unrealistic and amateurish and that their only result was to give Hitler even more reasons to distrust his generals, in a time when the military situation was dire. And, as we all know, this had catastrophic consequences for the German people.
