Zuikaku with Japanese at Midway

Carriers are a red Herring , the issue for the IJN is pilots/aircraft. Having one more carrier survive but lose more planes/pilots is a net loss. It just means a bigger sacrificial diversion later not any major impact on future battles.

IIRC actual pilot losses at Midway for the IJN was quite low (for some reason I'm thinking 80 odd) so the pool was hardly drained by that battle

What hurt them at Midway (other than 4 of their Fleet carriers) was the loss of the highly trained and competent 'ground' crew on those 4 carriers

Losses were Akagi: 267 (Complement 1630) ; Kaga: 811 (Complement 1708); Hiryū: 392 (Complement 1100); Soryū: 711 (Complement 1100)
 
I can imagine a scenario where the Japanese changed their codes and the Americans are intelligence blind for a couple of weeks late may Early June, so the Carriers are back at Pearl when the Japanese strike. ...

The IJN did change the JN25 code in March. This was the code used by the fleet & subordinate HQ. The tactical radio code was different (Altogether some 28 different codes or encryption systems were identified as used by the Japanese military during the war, including the diplomatic codes.)

The change in March was routine, both in that the IJN changed the keys for that code every 3-4 months, and preferred to change it during the run up for a new set of operations. This shut the USN intelligence out of the IJN HQ radio traffic for weeks. The breakthrough was aided by the Tokyo raid. When Dolittles bombers appeared over Tokyo the Japanese home fleet went crazy and set to sea like mice flushed out of barn feed bin. The radio traffic was at the maximum the operators could transmit, and because of the pressure sloppy and full of the sort of errors that code breakers love. within a few days the US code breakers had enough clues they found the keys and were back inside the JN25. Take away the Dolittle raid and it could have been weeks, perhaps months before the USN intelligence staffs were regularly reading the messages again.

... so the Carriers are back at Pearl when the Japanese strike. ...

Nimitz is caught with his pants down, He does a emergency sortie of the fleet, but not knowing the situation he is cautious. The fleet reconnoiters and probes from a distance. Midway is less well defended and the Japanese have the leisure to try a landing operation. Suceed or fail its a very bloody operation for the landing force. Odds are Yamamotos fleet fails to pin & destroy Fletchers small fleet. If they stray to floe to Hawaii they are harassed by the 500+ aircraft based there. Eventually Yamamoto returns to home base declaring victory after losing the better part of the landing force, and a ship or two from US airpower and a lucky submarine shot. The fleet has burned a immense amount of fuel for several weeks, and maybe gained a island it cant well support.
 
Without the Doolittle raid, Yamamoto doesn't get approval for the Midway operation.

In that case, and the southern option is the only approved option, Would the Japanese still only do OTL Coral sea in May with two Japanese carriers (thinking only two is needed, why would more American carriers be there??)
OR
Would they do some elaborate Midway style multi-task force thing in the coral sea, in June with all 6 fleet carriers and the 5 smaller carriers (maybe trying to take New Caledonia or some extra place)
 
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Would they do some elaborate Midway style multi-task force thing in the coral sea, in June with all 6 fleet carriers and the 5 smaller carriers (maybe trying to take New Caledonia or some extra place)

The existing Coral Sea plan of the IJN was complex enough, so my wild guess is yes.
 
Without the Doolittle raid, Yamamoto doesn't get approval for the Midway operation.

This. They only got Army Approval due to shock relating to the fact that the Emperor's life had been endangered by the Doolittle raid. Without it it's literally anyone's guess what they try next, probably some damn fool thing in the south pacific, with entirely too many moving parts, and doing it again and again till they finally get burned.
 
The IJA had one future aim in early 1942. Prevent Australia being used as a base to take back the ‘Southern Resource Area’. The IJA would not invade Australia but use the island chain to the east for air bases to interdict supplies. There was no appetite for expanding the perimeter westward. The IJN was working on supporting the IJA in its isolation campaign.

At this time the USN decided there was something to learn from Japanese carrier ops and resolved to operate in pairs wherever possible. Yorktown and Lexington were deployed south to counter the Japanese.

The IJN should have been aware that the USN would risk major assets in defence of Australia. After supporting the invasion of Rabaul with 4 carriers, they were embarrassed by the attacks on the invasion of New Guinea by Yorktown and Lexington and the defeat of the Land Based counter attacks on the carrier group.

The next move would be supported by carriers but after 6 months of continuous ops the carriers and crews needed refit and rest so only Car Div 5 was allocated as they needed more combat ops and experience.

The Doolittle raid completely transformed the IJAs mindset. Hawaii as a base for American naval forces could directly threaten Japan. This was the core of the empire, not the peripheral resource area. The IJA was now alive to what the IJN had being saying, eliminate Hawaii as a base. The new ‘Eastern Operation’ as a joint IJA-IJN goal was born.

The Allied intelligence assessment before Coral Sea had correctly identified the forces involved and correctly ascertained that it was a limited offensive. Had it been the whole KB then an invasion of New Caledonia would have been anticipated and a whole new level of risk and commitment.

If the IJN only wanted to eliminate US Naval forces as per the Midway plan then the drive south would have offered that and the USN wouldn’t have the advantage of a fully equipped fleet base close by. They failed to see that the South Pacific was the attrition battle that they had anticipated would be through the Mandates in pre-war plans. A mistake they continued to make into 1943. A combined IJA-IJN focus on the island chain campaign south should have been the focus for 1942. It also had the other advantage of being down the fault line of US Army/USNavy responsibilities. As it was they had to shift the line to get Watchtower approved as a USNavy op.
 
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