Ditching options such as small base facilities at FFS would improve survivability for the crews, but survivability is, like in the Aleutians, also subject to the bad weather conditions of the region for much of the year, and compounded by the long over sea flight profile. Probably a safer operation during summer months than winter months.
To better secure Midway the IJN can be expected to 'hop' down the small islands between Midway and Kauai with small destroyer borne invasion groups, while the Americans will be moving the other way. The situation might be a bit fluid until Nimitz cuts to the chase and invades Midway with a full Marine division to be done with it. But if he does that, then no Watchtower.
Not sure what the cruising speed of a B-24 was, but if 130mph were talking an 18 hour flight, so crew exhaustion and maintenance would be a factor. Bomb loads would be absurdly low because of all the fuel carried. Overall, targeting Midway would reduce operational tempo of each aircraft in comparison to use against a closer target, reduce the bomb load per mission significantly, and increase overall attrition. So, altogether, let's say half the bombs and half the expected operational life of the plane. .5x.5 = .25. Bombing Midway from Kauai would take a B-24 and give it maybe 1/4 as effective a combat career as the same plane used elsewhere for a closer target in better weather; the more planes the USAAF uses bombing Midway, the better for Japan.