Regarding any suggested submarine blockade of Japan
No suggestion about it, US boats were very effective at blockading Japan. RN boats were not suitable for the Pacific, with the possible exception of the "T" class; range and air conditioning being primary requisites.
The plan to rely on the subs was doomed, since we simply didnīt have enough at the time. Would there have been 500-600 of em in 1940,
There was no plan to rely on subs, the Wehrmacht reached the Channel Coast and then what? Doenitz aspired to 100 boats on patrol, 100 in transit and 100 in port, he had about 60 in total at the start of WW2. Perversely, 60 Type XX1 in 1943 could have defeated the UK (snorkel, hydraulic reload, ability to catch a convoy whilst submerged, ability to run under a convoy whilst submerged, ability to flee Fido whilst submerged, ability to flee most escorts whilst submerged). Several other factors to throw in eg cut the Enigma chatter, use FW200 purely for reconnaissance, JU88s to intercept Coastal Command over the Bay of Biscay. But the end result is the same: Soviet tanks roll into Berlin.
For Pearl Harbour, if the USN carriers were sunk it makes no difference to the end game: American industrial power destroys Japan. The concept of the Americans negotiating a compromise after PH is a nonsense.