Its easy really. The Initial IJA reaction force is the Ichiki Butai of less than 1000 men, which is not available, being dead at Midway. The next available force is elements of 35 inf BDE which is not landed until after Eastern Solomons 24/25 August. Nearly 3 months after Midway.
Ichiki's force would be dead, true (perhaps no great loss where Guadalcanal is concerned), but if indeed Yamamoto has shifted into full-scale FS planning, we cannot exclude that he would have begun a troop buildup in the Rabaul area as prelude to the operation, rebuilding the South Seas Force as necessary. I don't think we can say with confidence that available elements in the area would be the same as in OTL.
Which is plenty of time for Wasp, Sara and possibly RN carriers to redeploy Illustrious is in Ceylon and Victorious on Arctic convoy duties. Now would all of KB sortie after Midway or would it refit and retrain aircrew for what's next. The air groups would be severely depleted and the IJN tended not to merge groups of split Cardiv. Its not a given either way unless you accept that steaming all over, accelerating the need for refit and burning oil, not to mention operational losses to the air wings, is an inherent good.
Running slow transports during the second half of August means running them in the face of Cactus Dauntless with no docks at the far end. Without air cover they are targets, with air cover the carriers are targets and, again, no realistic air warning system on the IJN.
And that's why you do Watchtower if there is a reasonable chance of supporting the Marines ashore. Hell The RN can support Malta for years against greater odds with no chance of sinking Sicily what is the USN a bunch of pussies?
I don't think the "pussies" comment is called for here. As it was in OTL, Vandegrift (no shrinking violet he) and Ghormley were both appalled at the decision to go to Guadalcanal when they learned of it on June 24, because they simply didn't think the Marines and the logistics were ready (which was true in the absolute logistic sense, at any rate, since D-Day had to be moved back from Aug 1 to Aug 7, not least because of that ridiculous Wellington dock workers' strike); Vandegrift felt that months of training were needed in New Zealand, and had not anticipated any offensive action until the new year. In this he was vindicated, when he saw after hitting the beach how inept the patrol patterns his men were doing around the initial perimeter were. The marines would have to learn on the job, unfortunately, and they did; but I also think Vandegrift (proud as he was of his marines) would be the first to tell you that it was more surprise and Japanese ineptitude that kept the command alive in those first weeks.
But the more important point here is the posture of King and Nimitz, and what we can discern of what *they* (not I) would do here. Before Midway, all of King's efforts were directed to the buildup of forces in the islands securing the main lines to Australia and New Zealand, shipping 56,000-man garrison forces (41,000 Army; 15, 000 Marines) to New Cal, Fiji, Samoa, and Efate between January and May (most of them actually not arriving until May, giving King great anxiety). While the Solomons was part of the planning for an axis of advance from the start (even in early February, King was urging a garrison at Efate, as "it will serve to deny a stepping stone to the Japanese if they moved South from Rabaul, New Britain," and provide a strong point "from which a step-by-step general advance could be made through the New Hebrides, Solomons and Bismarcks." (a) COMINCH, letters, FF1/A16-3/F-1, Ser 00105 of 18 Feb 1942), the really decisive move by King for Guadalcanal did not come until
after Midway, with King's famous June 24 memo - when what was about to become WATCHTOWER was seen as a way of *keeping* the initiative in American hands after its great victory, rather than *seizing* it away from the Japanese.
In fact, in May MacArthur and King had discussed the possibility of sending a Marine battalion to destroy the Japanese base being built at Tulagi. This was seen as a raid rather than a garrison. But on June 1, three days before Midway, King backed away from the plan as too risky (CinCPac ltr to CinCSWPA, 28May42 (located at NHD)). He decided that the force was not big enough, even for a raid in force.
In a scenario where Nimitz has just lost three fleet carriers and the Kido Butai remains intact (albeit presumably with ravaged air crews) it is hard to see how you get the shift you see in King's stance in June. Rather, in OTL, it was clearly done in reaction to what had happened in the battle at Midway. If the Japanese juggernaut is still rolling, it seems far more likely that the pattern of January-May continues to assert itself: Continue the buildup of American forces in the islands of Melanesia against any further Japanese advance (hopefully now with Somerville's fast carrriers in hand), which now would be more likely than ever.