Re: Submarine max depth questions.
Another comment on crush depths - nobody in their right mind is going to let their submarine go below the test depth (i.e. that depth the shipyard says is safe and has been tested on builder's trials) voluntarily. It's only going to happen when you are being actively hunted by a competent ASW force, and it's your only way out. This happened to the Germans in the North Atlantic (very deep), the British in the Mediterranean (generally pretty shallow) and rarely to the Americans in the Pacific (the Japanese weren't very good). Hence, the overwhelming majority of experience of being hunted into very great depths will be on the part of the Germans.
Oh, and one other thought (think it's been covered before) - nobody will ever know that a submarine failed due to water pressure rather than depth charging if it was being actively hunted at the time it sank. It's still a kill either way, but is relevant when working out the hull strength...
I have neither the time nor the inclination to differentiate between the incompetent and the merely unfortunate - Curtis E LeMay