The Allied OB & reinforcement schedules were not set in stone. If a decision is made to execute Op DRAGOON in August, or whenever, the shipping schedules would be redone. It took a bit over 45 days for a Liberty Ship, the slowest transport, to make the round trip from a US port to Algiers, & less than two weeks to make the one way trip from Liverpool to Algiers. OTL reinforcements intended for the French Atlantic ports that were redirected to Marsailles arrived with a few weeks, or in some cases days delayl. Not months.
Ok, you are right that more can be brought in. You can e.g. get a convoy at 10 knots from England to southern France in about ten days. The problem is that when you do it, along with bringing in some 200 LST ships and hundreds of LCT boats from the north, if you want to keep some pace of reinforcement, you have to get through Gibraltar and when allies did that, Germany immediately learned about it. So no surprise there.
The other half of this is the state of the German ground forces after this Normandy victory. I suspect the Allied Joint Chiefs are not going to order up this if they don't have have assurance the Germans are badly attritioned. 10+ fresh Allied divisions can deal with worn enemy formations trickling down from the north.
That's the point. They wouldn't be trickling from the north, they would already be in the south. The scenario (though unlikely) was a rather quick defeat of the landings, so not much attrition of German panzer forces (the only really effective anti-tank forces) except for the three panzer divisions available in Normandy on the D-Day repelling the landings. Considering that if Overlord was defeated quickly, there would be no need to transfer the two panzer divisions from the south. I.e. Dragoon probably faces at least the three German panzer divisions (9th and 11th Wehrmacht and 2nd SS) in the south which were located there on the D-Day.
Ah, but DRAGOON of OTL was supported by 3000+ aircraft. The US had over eight months to lay out airfields, and prepare for a invasion surge of additional temporary units. As I mentioned earlier, between the securing of Corsica in November 1943 & early January 1944 close to 1,100 fighters and bombers, along with some reconissance & ASW aircraft were permanently based on Corsica alone.
My argument was not against the overall number of planes available for Dragoon. My argument was that there were not those numbers available in the same vicinity to the landing beaches as they were to Normandy. The only airbases with comparable distance to the invasion area would be those on Corsica (Even Sardinia is already nearly twice as far from southern France as Normandy is from southern England).
Beyond that the heavy bombers of the 8th AF & their escorts were ranging further in 1944. Fghters from the UK can be attacking German airfields in south France similar to Germany.
Well, that's true but that is a completely different pattern of aerial operations. They flew there, saw the heavies bomb and returned. They didn't spent there hours of loitering over invasion beaches to repel a possible German attack. The point of fighter cover over the landing area is not to allow enemy bombers and fighter bombers attack your invasion fleet and since it is the enemy that chooses when to attack, you have to patrol all the time during the day. Even in Normandy with far shorter distances (and 3-4 thousand fighter planes) the allies were not able to keep above the beaches more than one third of the time and that with great exhaustion for the pilots (remember how many sorties Clostermann flew on D-Day?).
Even under fairly conservative estimates that adds up to some 300 fighters available for CAP in any particular daylight hour.
I agree with that but that would still not be enough. Even if the allies really operated 1.100 fighter planes from Corsica and hundreds more using Corsica just for refueling, they wouldn't be able to keep more than a third over the beaches at any time. That would be indeed be the number of fighter planes over the beaches you say (perhaps even more). That is a lot but not enough and it would have great problems against a concentrated German air attack of the Bodenplatte style. Notwithstanding that if you use all Corsican and Sardinian bases for fighter planes providing CAP over the landing area, you don't have space for fighter bombers which were crucial for the success of the Overlord. Because of distance, they can't simply operate from mainland Italy or north Africa.
Unless you acquire first the nearer airbases in north-western Italy, you will not be able to recreate the same degree of air support the allies had in Normandy. It's an issue of distance and availability of nearby bases. On the other hand, capturing northern Italy was in allied powers in the fall of 44, so why not trying it in August 44 instead of Dragoon? With that you could carry out a much stronger Dragoon in say November 44.
As with the ground forces one has to consider the state of the German air forces after a battle in Normandy. Through restriction to night operations they managed to peak at a bit over 1,300 bomber sorties in early July, but operational losses alone ran that down swiftly. That peak number was reached by removing bombers from Italy, the Balkans, Norway, & the replacements from Germany. Whats left OTL in late July was unimpressive. By the time of MARKET-GARDEN in September surge strength of 200+ bombers at night were about all that was practical. To return to the BODENPLATTE example the initial strength was under 500 bombers, and between undertrained pilots, worn out or badly built new aircraft accident rates were reaching above 40% of sorties. The necessity to attack at night to avoid combat losses meant abysmal accuracy for the undertrained German aircrew.
Well, the premise of the scenario is a quick defeat of the landings, therefore neither Luftwaffe nor ground forces worn out by a prolonged two-month fighting. If the allies were still being mopped up in Normandy in August as you mention, it would be a very different scenario and Dragoon in August against German forces still engaged in the north would make sense, but then we are both describing different situations.
So with relatively low exhaustion on the side of Luftwaffe, you have a possibility of a German concentrated air attack against the Dragoon landing fleet. Hundreds of German fighters sent against the 300 or more allied fighter planes providing CAP over the beaches would be a big distraction and the 500 bombers you mention, despite suffering heavily from AA fire, would be a serious threat for the LST ships. The LST ship was a logistical bottleneck for the allies, as they were the only means of delivering substantial numbers of vehicles ashore over the beaches. In Normandy there were some 230 of them bringing in vehicles from the distance of 200 km. The distances in the south would be far greater and any incapacitated LST ships would reduce the speed of reinforcement substantially.
Concerning the possibility of bringing soldiers directly through the ports of Toul and Marseilles, well, you have to have them first and undamaged. In OTL they were captured in the end of August against already withdrawing forces whose main panzer elements were sent to Normandy in June. In the ATL the advance would not be that fast, Germans substantially stronger on the ground and allied fighter and bomber forces weaker. There is a reasons to assume that both ports would end up rather like Cherbourg. As a consequence, most of the troops and vehicles would still have to be brought as in Normandy over the beaches by LSTs which themselves would load the troops either in Corsica, Sardinia, North Africa and Italy with the port capacity in Corsica and Sardinia being rather limited. That creates the LST bottleneck mentioned above.
Unless the Allied Joint Chiefs go collectively crazy they are not going to execute a second innovation in 1944 unless they have some assurance it has a chance. This leads us back to the unanswered question of why Op NEPTUNE or OVRELORD fails.
I agree that the reason of failure of Overlord would be paradigmatic for whether or not they would try again in summer of 44. If it was very unlucky weather, that could be corrected. If failure of amphibious tanks, that too. However, they couldn't wait for ever and the window of opportunity in the north (unlike in the Mediterranean) would close around September. Again, it's hard to imagine a failure drastic enough to warrant a complete withdrawal from Normandy, but this is a theoretical search for plan B for a critical scenario.
OTL the Germans had information on the Amphib fleet returning to the Med during July. Allied deception ops had them attempting reinforcing Italy from Genoa southwards, the Adriatic coast, Dalmatia, and Greece. They also took the Allied deception op aimed at Bourdeux seriously. Hence their Pz corps in AG G being out of position to deal with the landing on the Riviera.
The reason why the two of the three panzer divisions were sent northwards was Normandy landing. If that failed quickly, there would be no reasons for them to continue northwards, or not to return. Attacking in the Bay of Biscay would represent even greater problem from the perspective of fighter cover of the invasion fleet than Dragoon (in fact even northern Brittany would be a nightmare for the allied fighter forces). Also, bringing hundreds of LSTs and LCTs (without which there would not be a major invasion in the south other than the one of the size and low speed of Dragoon landings in OTL against weakened and already withdrawing forces) plus hundreds of cargo ships full of soldiers (in Torch one division minus non-combat personnel required a whole convoy) would alert the Germans overlooking Gibraltar. The cover up you describe was only possible because in OTL there was no such massive transfer of forces from north to south. Dragoon was carried out with forces already available in the Mediterranean.