When should Germany have surrendered?

[Grumbler]

First, there are these thing called boats. Small boats evacuated the 300,000 from Dunkirk. There are as many small boats in Europe as there are in England. Because they are small, you can load them on trains and move them to the ports that are needed.
As far as the RAF, controlling the channel, they couldn't in 1940. The Germans destroyed the coastal convoys that tried to use the channel and there was nothing the Royal navy or air force could do to stop them. The admiralty gave up on coastal shipping. The RAF won the battlke of Britian because the Bf-109 didn't have the range to fight over London, it did over the channel. That is why the RAF got spanked over Dunkirk and was unable to prevent the coastal convoys from being slaughtered.
The Luftwaffe had control of the air over the channel by late summer of 1940. They could not extend that control to cover lower England, but you are wrong to think the royal navy would have been able to prevent reinforcement of an air drop to capture a port capable of landing supplies and more men. The Germans could not have landed armor, but they didn't need armor to beat a bunch of old men and boys armed with longbows shotguns and pitchforks. Remember what happened to the Army and Navy at Crete?
As far as Donitz as a source, he always told Hitler what Hitler wanted to hear, why on earth do think he changed because he was a prisoner? Suck ups are suck ups. he boss can change but the sucking up doesn't.
And as a point of fact, the Japanese used 2nd line equipment in the Burma theater also. No zeros EVER fought in that theater. IIRC, there were Spitfires station at eother bangcok or Ragoon. They were shot out of the sky by the Japs in Ni-97's.
The Spitfire was a good plane, but it was far from the best plane. It was perhaps equal to the BF-109E. More 109's were lost by running out of fuel then were shot down by the RAF. Neither fighter was the equal of the Zero.
I say the Battle of the Atlantic was won in April of '43 because that was when the Allied production of shipping exceeded loses of shipping for the first time. And as you pointed out, WW2 was a war of production and logistics.

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The Fallschirmjagers took almost 50% casualties. Crete could have been any small British coastal town with a dock. I think turn around time from fwance to england would have been shorter, which means more then two sorites could have ben flown. Air drop about 2 AM, then again about 5Am, with a 3rd wave either landing on captured airfilds or, if that didn't happen( unlikely, since it would require some British airman with an Enfield with 4 rounds fighting off a few dozen Fallschirmjager armed to the teeth)
The RAF had maybe a handfull of night fighters at that time. The greatest danger to the Germans would have been midair collision. The Raf, might have gotten too the 3rd wave, but by then they would have been better used against the small boats full of troops landing on the beaches.
No, the Germans could have invaded England in September of '40, if they had wanted to. The didn't want to, so they didn't. It was nothing the british did or could do that stopped them. Burning OIL on the water? So what? the boat will get to the beach before it's damaged by the burning OIL.