Hi guys,
Haven't posted here in a while, but since we are talking once more about a subject dear to me, allow me to contribute some thoughts.
I haven't understood exactly the premise of this TL, does the opening post asks for Guadalcanal fought with IJN having all four Midway carriers available (Akagi, Kaga, Hiryu, Soryu), but with the US having lost the same number as OTL (Lexington and Yorktown)? Or is it with the americans having those two CVs surviving and available as well?
If the former is the case, if the americans still carry Watchtower they must think they won "something" previously to feel confident to carry such an operation in August 1942.
One of my "simple" scenarios for this is Hara and Takagi do not send the disastruous dusk strike on the 7th of May so they don't lose 9 of their best crews (8 kanko, one kanbaku), so next day they have those extra planes to use against Fletcher, Yorktown is more seriously damaged so it cannot be at Midway, but is sent for repairs. The americans will think they sunk "two" japanese carriers to one of theirs.
Midway then happens two vs four, let's give Nagumo a bit more luck so we have TF 16 discovered by the Chikuma 1 scout at 0630, so then a strike is sent either before 08.00 or before 09.00, we have 34 kanbaku and 36 or 43 kanko (if Kaga launches it's full complement) with about a dozen Zeros, the americans have a lot of CAP comparatively though (about 30, as 20 of those were escorting the strikes), so the japanese strike takes heavy losses, but they cripple one and damage another US CV. TF 16 is out of the fight.
Meanwhile, let's assume Ring still goes who knows where, that only leaves VB/VS-6 to attack Nagumo, a wee bit of luck for him and the SBDs are spotted a bit earlier, and with just two TBD squadrons to contend with there are more Zeros with ammo and available to go after the SBDs which are serioulsy disrupted in their attacks, let's say they hit only Kaga (but miraculously, couple of bombs hit the fantail or bow or at least in places where there are no armed and fueled planes - only the planes that returned from Midway are onboard in this scenario, just 17 kanbakus), while Akagi is only near missed by Best or someone else (but they report it as a hit) Now we have subsequent KB attacks sinking the crippled carrier (say Hornet) while the damaged one somehow escapes attention and retires away from the battle. At this point, let us say that based on reports the americans think they sunk or damaged "two" or even "three" IJN carriers ("one" by Midway B-17s and/or SBD and "one or two" by the CV-6 SBDs).
After this, let us say the japanese do shell Midway during the night, seriously affecting the airfield and surviving planes, maybe the next day KB sends another attack from it's 3 intact CVs, then they try the invasion anyway on the 7th, but as some here believe it eventually fails (they lose a few transports in the process), which greatly adds to the american impression that they "won" something. The somewhat dejected japanese call it a day, return home, get to repair Kaga and reorganize the airgroups which have taken losses (i guesstimate in this scenario, about 50-60 planes and about 100-120 aircrew- most of the aircrew losses happened in the attack against TF16) obviously not a disaster but coupled with Coral Sea they will need time to reorganize and train.
So fast forward, the relatively moderate damage to Kaga is repaired for it to be available at Guadalcanal (but for realism, we could have it more severely damaged at Midway and still under repair at this point), the confident americans (with 3 carriers available, plus one at Pearl just like OTL, or still under repair to to be available shortly) about to be met by no less than six (!) KB carriers, a massive change and boost for KB. If the americans still fumble around like in OTL chances are good the US carriers are sunk or damaged in August, and their troops on the island will find themselves in a much worse situation compared to OTL. If IJN controls the waters around Guadalcanal, their convoys will take much fewer losses and more troops and materials get to the island, so it's very possible the americans lose the airfield and/or are evacuated by September-October, which significantly changes the Pacific war in late 1942 and into 1943. Will the freed up men ships and planes thrown at Port Moresby after October change the balance there? Even if the americans still hold PM, they will advance much slower in NG, with many more casualties compared to OTL (because many more japanese troops and materials are available for NG).
Come the end of 1943 when as some of you point out the US carrier fleet is rejuvenated, will they still go for isolating Rabaul (which means basically starting the Solomons campaign in late 1943), or they will go straight for Central Pacific? Whichever case, they will have to face a much stronger Kido Butai probably in a battle royal in late 1943, admittledly KB's airgroups could still be weakened by repeated assignments to land bases for operations (in this case in NG), but perhaps not as bad as in OTL, so you have about 9 US carriers versus a similar number of IJN ones. As Glenn points out, by this time KB will be facing grim odds, but at least it will not be nearly as bad as at Marianas, they could still dish out some serious damage to the USN in one last hoorah. At least it would be a more fitting demise for the mighty Kido Butai.