I see you refer to Overlord I a lot of times but that is completely misleading.
Well, I realize after reading your post that I left out some details that supported what was supposed to be my thesis: that having guarding forces on ports and to the flanks of ports is a good idea.
Things I left out of my post:
Overlord I was reacted to swiftly and decisively in a way Overlord II was not. During Overlord I, Guderian's panzer army was not on the front lines and was resting in Germany. When I saw the troops land, Guderian's panzer army, plus the 4 panzer divisions assigned to cover the west immediately went to contain/destroy the landing (along with some other forces). The delays caused at the invasion points were significant enough that the landings in Spain never really gained a foothold. The two landings in Spain took only one of the two required ports (they were in different regions in Spain), and unless I'm going senile, even the landing that took a port never made inroads enough to give the landing divisions room to retreat and regain ORG, which is in line with my previous statements. (Kudos to the Italians for a tenacious defense in the south.)
The landings in Jutland did gain a foothold and a port, but I also detailed more reaction forces there to stop the attack as I was more threatened by it. It would have been a dagger pointed at the heart of the Reich if the Axis didn't kill it.
Overlord II did not receive the same treatment, as I did not take it seriously. Guderian's panzers (or Rommel's, for that matter) were never sent west to crush the invasion. The other forces were sent too late for make a difference. I also lost the four panzer divisions pointlessly in Brittany. So, the defenders bought time for no one to come save them.
It's not a flaw in the defense plan to use those forces to slow the enemy down; it's a flaw in the execution where I don't take the British Army seriously after what happened in Italy, Syria, and Overlord I. I won't be making that mistake ever again, especially when Italian intelligence was telling me that Britain had swapped to the training continuous focus before they landed.
It's objective was never the complete downfall of Germany. At best it was hoped that a sizeable force could be cut off and destroyed and those factories in Spain denied to you.
Accomplishing the liberation of Spain and Portugal would have defeated me. Under 1.3.2, my tanks were driving to the Ukraine and Caususes with tungsten instead of oil. It might still have taken awhile, but that was a nice chunk of IC and resources to build the mission critical equipment of the war.
That's actually one psychological reason I may not have taken Overlord II seriously. If the tungsten was not in immediate danger, my anxiety level was lower.