If the British had held onto Singapore then Luzon could have been supplied far more easily than trying a relief operation from Oahu.
How? The British had great difficulty just projecting power out to Singapore, given that Europe and the Med was absorbing the preponderance of their resources, so how are they then supposed to mount an offensive to secure the SLOCs then? Are you proposing that the US rebase the entire Pacific fleet to Singapore? Because the logistics of achieving that'd probably take just as long as the historical strategy (if not longer), which means Luzon still falls long before the USN can get there.
You stated that fighters were not on Oahu to ward off carrier attack. Yes, they were.
Your own source says that it was the bombers who were there to ward off carrier attack, which they obviously failed to do. For the fighters, it says they were there for training:
"Superficially, Oahu's needs for pursuit craft appeared much better met. During most of the time between May and December 1941 it had about 150 Army pursuit and fighter planes, two-thirds of them modern P-40's.
But a chronic shortage of spare parts kept many of these planes out of commission, and the ones available had to be used intensively for training. The greatest qualification was that pursuit planes, however modern, were all but worthless as defense equipment in the absence of an effective warning system, and Oahu had none before the attack on Pearl Harbor."
When your own sources are contradicting you, it's probably best to stop digging.
Giving the Japanese 8-12 months to fortify Iwo Jima and Okinawa increased casualties and missed the opportunity right after the victory at Marianas. The USN was plodding and deliberate on the offensive, rarely showing any signs of opportunism.
Spending 8-12 months blockading and bombarding the forces on those islands deprived them of much of their supplies, denied them any prospect of reinforcement, and reduced their strength, while allowing the US to marshal additional forces for the attacks, all of which probably reduced casualties. The USN on the offensive was moving just as fast as it should have been. As was always the case in the Pacific War, the passage of time saw US forces grow stronger while those of the Japanese grew weaker.
The major error the Japanese committed in the South Pacific was to exhaust their airpower and to overcommit ground troops that needed to be on Saipan and in the Philippines. That is to say, their errors after losing the Guadalcanal campaign were more serious than the ones in the SPO before that point.
The exhaustion of air and ground power was indeed an issue, but the commitment of naval forces also badly sapped Japanese strength there. Their errors during and before the Guadalcanal campaign were probably just as serious, if not more so, then those afterwards. Forcing the combined-seas fleet to chase the Americans hither and yon not only burnt up scarce fuel, the actual combat also consumed ships at an unfavorable rate to the Japanese and badly hurt the Japanese navy even before the USN's proper island hopping campaign got under way. The impact upon the merchant marine was also pretty bad: Guadalcanal cost the Japanese Merchant Marine 200,000 tons of shipping. That's not just 200,000 tons of shipping sunk, but 200,000 tons of shipping sunk in the ass end of the Pacific while it was doing nothing to bring back the resources that Japan needed, which would have been a much bigger contribution to the Japanese war effort then trying to shuttle troops and supplies to a doomed battle.
The main effect the South Pacific campaign had, from beginning to end, was that it allowed the US to be able to grind the Japanese forces down piece meal even before it had built up the strength for the main offensive into the western pacific. The entire concept of the perimeter was shown to be militarily bankrupt. Japan could not support the forces at the island perimeter, could not defend it, and proved unable to anticipate attacks on it or effectively respond to them. It was a strategic millstone.