Great report! And interesting question on whether or not two hits would sink the carrier.
Shoho was a pretty small carrier at about 11,000 tons, plus it was a conversion of a submarine support ship hull that was most likely built to naval auxiliary ship standards that could have been less survivable than warship standards.
The historical events at Coral Sea in May 1942, won't provide much insight on whether two dive bomb hits are enough to sink the small carrier, because of the overwhelming strike against it. According to Morison's history (Volume 4), the US strike was launched against what was thought to be the main carrier force, and when the 93 US aircraft arrived they found Shoho instead. Again according to Morison, the attack began at 1125 (other reports suggest 1115), Shoho took two 1,000 pound bomb hits, burst into flames and went dead in the water, more bomb and torpedo hits followed immediately, 20 or more hits being made in rapid succession, some reports suggest 7 torpedo hits and at least 14 bomb hits. Abandon ship was ordered at 1131 and the ship sank five minutes later. Would the first two hits have been enough to sink it? I would say probably, but let's look further.
The next day in the main carrier battle Shokaku took two bomb hits and was burning furiously when it took a third. The ship was a mission kill and put out of action, but it survived and nearly sank while returning to port for repairs. Shokaku at more than 25,000 tons was more than twice the displacement of Shoho, and was built to warship hull standards after expiration of the Washington Treaty limitations. Three hits caused a mission kill on a 25,000+ ton carrier and would almost certainly have caused at least a mission kill on the much smaller Shoho.
Also, during that engagement, Lexington, a 36,000 ton ship built on a battle cruiser hull as a result of Washington Treaty limitations, took two torpedo hits, then two bomb hits plus near misses, and was ultimately scuttled by destroyer torpedoes. Yorktown, at almost 20,000 tons and built to warship standards, took one 800 pound bomb hit, continued flight operations, and returned to Pearl Harbor for repairs before its fateful destiny at Midway in June.
Looking to Midway for more examples, Akagi, 36,500 tons standard displacement, was built on a battleship hull as a result of Washington Treaty limitations, took three 1,000 pound bomb hits, the ship was abandoned, and before dawn the next day was sunk by torpedoes from Japanese destroyers.
Kaga, 38,200 tons standard displacement, was also built on a battleship hull as a result of Washington Treaty limitations, took four bomb hits, the ship was reduced to a flaming wreck and sank that evening. Gallaher's squadron attacked Kaga and was armed with 500 pound bombs because with a full deck load, his squadron at the front of the line and didn't have enough take-off run to gain enough speed to lift a 1,000 pound bomb. A division of Best's squadron also attacked Kaga, armed with 1,000 pound bombs because they had more take-off run and could gain enough speed to lift the heavier bomb. Planes from both squadrons hit Kaga, the mix of bomb hits has been reported as one 1,000 pound bomb and three 500 pound bombs. After the ship was abandoned, it was scuttled by torpedoes from Japanese destroyers.
Soryu, a smaller ship at almost 16,000 tons, was designed from the keel up as an aircraft carrier, took three 1,000 bomb hits, the entire ship burst into flames, and within 20 minutes the crew was ordered to abandon ship. Damage control parties had been sent back aboard, and fires appeared to be under control. US submarine Nautilus closed on the ship, fired three torpedoes, all hit, a periscope look two hours later after surviving a prolonged depth charge attack revealed the ship burning along its entire length. The ship finally blew up and sank three hours later. Later research suggests that the scuttling coup de grace was delivered by destroyer torpedoes.
Hiryu, a modified Soryu class, at just over 17,000 tons, was designed from the keel up as an aircraft carrier, took four 1,000 bomb hits, and the ship burned for hours, was torpedoed by Japanese destroyers about 12 hours after the dive bomber attack, continued to burn fiercely for another four hours, and then sank.
Yorktown, damaged at Coral Sea, took three dive bomb hits, then the slowly moving carrier took two aerial torpedo hits. The ship was abandoned, then later boarded by small salvage parties, then still later by a larger salvage party brought by US destroyer Hammann. After penetrating the protective destroyer screen, Japanese submarine I-168 fired four torpedoes, two going under Hammann and hitting Yorktown and one hitting Hammann. Hammann sank within four minutes, Yorktown, further damaged by Hammann's depth charges exploding as they sank with the ship, stubbornly remained afloat and finally sank the next day.
In tabular form:
Data is in the following order: Ship, Standard Displacement Tons, Dive Bomb Hits, Aerial Torpedo Hits, Submarine Torpedo Hits, Scuttled
Shoho; 11,200; 14+; 7; 0; Sank not scuttled
Shokaku; 25,700; 3; 0; 0; Mission kill, not sunk
Lexington; 36,000; 2; 2; 0; Destroyer torpedoes
Yorktown; 19,800; 1; 0; 0; Not sunk
Akagi; 36,500; 3; 0; 0; Destroyer torpedoes
Kaga; 38,200; 4; 0; 0; Destroyer torpedoes
Soryu; 15,900; 3; 0; 3; Destroyer torpedoes
Hiryu; 17,300; 4; 0; 0; Destroyer torpedoes
Yorktown; 19,800; 3; 2; 2; Sank w/o scuttling
So, the two cases closest to two hits on Shoho would be Shokaku and Akagi with three dive bomb hits and no torpedo hits. Both of these much larger ships, designed as warships, experienced mission kills, Shokaku survived and barely made it back to port for repairs. Akagi could not be saved and was scuttled by destroyer torpedoes.
Based on this damage summary, I would say that two dive bomb hits on the small Shoho, a conversion of a naval auxiliary ship, would have almost certainly been a mission kill, and most likely would also have resulted in outright sinking.
However, just to prove that nothing is ever almost certain, or even most likely, in warship damage, the ship closest in size to Shoho, the Soryu, a warship design, took three bomb hits, three submarine torpedo hits, and destroyer scuttling torpedoes before sinking, and Hiryu, a modified Soryu class ship, took four bomb hits, destroyer scuttling torpedoes 12 hours later, and still survived for another four hours after that before sinking.
Short answer: Yes, two dive bomber hits would sink Shoho. Now waffling a little, two hits might not sink it every time, but if the rule set provides for most likely sinking the ship on two hits, I would say it is historically accurate. If two hits always sink a Shoho type aircraft carrier, I might change that to make it less than 100% certainty, but not a lot less. If two hits is always a mission kill on a Shoho type aircraft carrier, I would say that is historically accurate.
