Hitler overruled his general staff quite commonly. The most common one brought to the topic of this is typically that of the Battle of Stalingrad where Hitler overruled Erich Mannstein's request to attempt to relieve the encircled German forces after Operation Uranus which was famously turned down by Hitler. Though, it should be noted that in such an instance where the Germans had lost the capacity of one of their armies, that the Wehrmacht had effectively one less army to fight with in what was turning into an up-hill (Or more accurately, up-mountain) battle. What also should be noted is that in this instance is that the Wehrmacht had two armies still fighting in the Caucasus in the time Uranus had occurred, meaning that even if they could evacuate the Sixth Army from Stalingrad, the army that would relieve them would be worn down from moving in to evacuate risking the 1st Panzer Army, the 17th Army, and the Wehrmacht's own defensive positions in the Ukraine. At this point, you have to consider what would be more important, saving one army at the risk of the entire Southern Theater, considering it was risky at best that they could even relieve the defenders, or two armies at the loss of potentially saving one. Overall, that decision, albeit a difficult one, was vital to the war effort from the Axis side of things and part of why the troops in Stalingrad were ordered to remain is that Stalingrad was still an important hub in terms of infrastructure for the Southern Russian front and by having the troops in Stalingrad continue to fight would consistently deny this hub.

It should be noted that many of the Wehrmacht generals who weren't executed Post-War typically wrote their own memoirs of the war and these memoirs would typically be years old from memory and therefore very distrustful as there can also be the instance of trying to romanticize or up-talk their own involvement within the scope of the war. Hitler's overruling of generals was not limited to matters of combat, however, but to matters of state as well and should be reminded that Hitler was a dictator with an overreaching part into all things of State. The rate at which Hitler turned down military operations could very well be comparable to those of Stalin or Churchill's own but as Hitler, thankfully, lost the war, his are seen as the big mistakes and are therefore more often popularized.

To answer your statement on whether this caused the July 20th Plot, the answer is more varied as many of the officers involved were affected by said overrulings. However, what ultimately caused the July 20th Plot was not so much as a desire to get rid of Nazism (But willing to do so if it meant peace with the West) but more to make peace with the Allies so that they could win the war against the Soviet Union which was seen as a "Crusade Against Bolshevism" in their "Drang Nach Osten (Push to the East)". There were elements in Hitler's command who felt that fighting the Allies would no doubt be a lost cause as already Operation Sealion had failed before it even took off and now that America was in the war, the cards were massively stacked against them as now 2/3 of the world's top three naval powers were at war with them effectively cutting off any chance of trade for war materials that the Axis could utilize. This, combined with the 24 hour bombings of Germany from Allied Air Squadrons left Germany in a position where her economy just simply couldn't keep up with the war's necessities. As such, would you not wish to make peace in this scenario?

As for Erwin Rommel's involvement in this plot, it is generally agreed to some extent that he had foreknowledge of a plot but did not participate in it though he may very well have agreed with them on this stance as he never reported it. The reason he was coerced into suicide is that someone within the plot simply said he was involved while under "investigative torture" in which Hitler would have taken him to the "Peoples' Court" which was in itself a death sentence for himself and his family if that happened, regardless of whether he was innocent or not. Therefore, when given the option to do suicide or court, he chose suicide as he saw that as the best option for his family.