You make all good points each and every time.That was the $64 question.
The 20th July plot was evidence that support for Hitler was wearing thin enough for the Heer to revolt. In 1918 enough of the German army melted away to create the Durchstoss legend. The only way to test an enemy's resolve is to attack and see what happens.
Some of the works on Op Market Garden point out that during the first week of September the German will to resist was very low and there was nothing to stop the allies going anywhere. But, by 17th September the Germans had recovered their nerve. Had Op Market Garden been launched on the 7th rather than the 17th of September alles might have been kaput.
Been a pleasure Mate!