I'm working on the basis that those who killed Hitler are even more likely to fail than the Valkyrie plot a year later, when the plot was far better organised and the war situation was even worse.
In general I agree, but the problem is with Hitler, Goering, and Himmler dead, the only Nazis with control over guns in a HIGHLY centralized state are dead; the opposition to the coup is headless, while the army, now severed of any loyalty to the Nazis, even if caught flat footed, isn't going to want to be led by any of the remaining Nazis.
By all accounts, Speer had a decent relationship with Goebbels until the last months of the war. I'm fairly sure that Guderian would have gone along with such an arrangement had it meant that he would have been able to increase his military and political capital.
Speer being friendly with Goebbels means nothing in terms of a post-Hitler government. Goebbels is useful or he's not. Also Speer had no guns under his control. Guderian also had no guns under his control, was being bribed by Hitler to accept whatever he said, and was more pro-army than Nazi.
As an aside, your Mao quote is rather selective. Whilst he did state that political power comes from the barrel of a gun, his broader point was that it was necessary for the CCP to be wedded to the PLA. Even then, he pointed out that the party must control the gun, and never the other way around.
Right, because the core point was that without control over guns you have no power; that means Goebbels, without any guns under his authority, had not actual power.