Singapore will ITTL be the base for one of the two major fleets facing the Japanese, with Pearl Harbour being the other. The largest fleet will be the American Pacific Fleet, while the British Far East Fleet, will concentrate on operations in the South China Sea, and around the various British and Dutch islands. The major problem will be when the Americans request facilities for their relief of the Philippines fleet. Yes Guam is roughly the same distance from the Philippines as Singapore is, however Guam is an undeveloped island in the Pacific, and Singapore has in addition to one of the worlds largest natural harbours, and a significant local industrial base to draw on. It might be much further away from the Continental United States, but it will be able to draw on shipping from both the east and west coasts, plus what ever the Empire can spare. You could even see a two pronged attack from both Singapore and Guam.
As a base for operations against the Japanese in the West Pacific or China Sea, Guam has three problems:
1) It's a small island with no infrastructure
2) It's a
long way from anywhere important - 1,000 miles east of the Philippines and 1,500 miles south of Japan - and even further from anything that could be call a logistical base (best part of 4,000 miles from Pearl Harbor)
3) It's almost certainly going to be occupied by Japan in the early phases of the war.
All through the inter-war era, the USN planners were grappling with the problem that in a war with Japan, the Japanese would occupy the Philippines and Guam, thus depriving the USN of its bases in the West Pacific. And without a base in the West Pacific, the USN could not support major fleet operations in Japanese or Philippine waters. (There weren't enough tankers to support a fleet over those distances, and if every damaged ship had to go all the way to Pearl Harbor for repairs the US Pacific Fleet would quickly lose its margin of superiority over the IJN.)
I was visualising a major campaign in Thailand and China where multiple corps of the US Army could potentially deploy. Why would the Army not want to win the war against the Japanese with the US Navy doing what its meant to do (giving the Army a ride to war)! It has the added benefit of directly aiding the brave Chinese in thir fight against Japanese aggression.
However Ramp Rat's right. There where good reasons why the Pacific war played out the way it did.
It's worth remembering that in 1941 it wasn't clear whether the "island-hopping" strategy would be logistically feasible. There were plenty of people arguing that recapturing the Philippines - let alone invading Japan - would be impossible without control of a developed port somewhere in the vicinity, and the only developed ports anywhere near were on the Asian mainland, so the first order of business would be a land offensive into Southern China to gain control of a port like Canton or Haiphong. There was even a plan for the US Army to fight its way the entire length of the Chinese coast and round the Yellow Sea, finally invading the Japanese home islands from Korea!
It never happened OTL, partly for political reasons, partly because Japanese advances in China and SE Asia deprived the proposed operation of its bases, partly because supplying an army in China via the Burma Road turned out to be an even harder logistical problem than supplying an army in the Philippines from across the Pacific and partly because the success of the island-hopping Central Pacific drive rendered it unnecessary.