I don't think the US would have fared that badly in war against Japan and Britain. The US Navy was always world class from the 1890s onwards, and was on par with the German and British Fleets. Furthermore the US, as demonstrated in WWII had a much greater ability to build additional ships, and US commanders were far more tactically agile. The US strangled Japan in world war II with a submarine campaign, and its hard not to see the US do the same.
The British and Japanese might get off a few good early wins, but they could not defeat the US, and both would likely begin to starve by the end of the second year.
You're being absurdly optimistic about how well the U.S. would have fared in a war against Britain c. 1914
As of the outbreak of war, the USN had 10 dreadnought battleships in commission, 4 dreadnoughts under construction, plus 23 pre-dreadnought battleships, with some of the older ones not really useful except as training vessels.
In comparison, the Royal Navy had 20 dreadnoughts in commission, plus 2 others that would be seized from the Turks and put into commission in August 1914.
The Japanese Navy would have been a much easier target It had 2 dreadnoughts in commission (with an additional 4 under construction), 4 semi-dreadnoughts, plus a variety of pre-dreadnoughts (including some ships captured from the Russians during the Russo-Japanese War that had been obsolescent then). However, British ships operating from Japanese ports in conjunction with their Japanese allies would have been able to take control of the Pacific.
The British decision to only finish those battleships near completion and not lay down new ships reflected the reality that the Royal Navy did not need more ships at the moment. If it had needed them, it had the capability in place to at least match and probably exceed the American shipyards in construction of capital vessels.
As for submarine warfare strangling Japan, the submarines of the era were by and large not capable of the endurance needed to hunt down Japanese vessels in Japanese waters. The first U.S. submarine designed for something more than coastal defense was the L-1 which went into commission in 1916.
Be that as it may in a conventional conflict, it bears remembering that the major reason Palmerston refused to intervene in 1863 in the US Civil War was fear of US Commerce raiders wiping out the British Maritime Fleet. Submarine warfare is exactly the sort of thing that would occur to the US in this scenario, and add to that a likely seizure of Canada.
Britain would have an enormous amount to lose from the war, even if it maintained naval superiority in the Atlantic, and the record both of its actual performance in the first world war(Jutland) and its amphibious assaults does not give much comfort that it would able to do more than keep the US Navy hemmed in. The US coast would be far too long for an effective blockade, landings would be bloody fiascoes which would simply get the British bogged down in guerrilla warfare, and all the time their merchant marine would be massacred.
At the same time you are forgetting that at this point the US industrial output was already more than twice that of Britain's, and far less dependent on trade and imports. It would have done far better in the war.
I actually will change my guess. You are right that the geographical limits of ships would probably make the Japanese safer. It would be the British who would be economically crippled.
You also forget the likelihood of a US sponsored Irish uprising as well.
That said, its for all of these reasons that i find the idea of this conflict implausible. Britain had so much more to lose than Japan from a war, and absolutely nothing to gain. the Japanese alliance wasn't worth one tenth that much, especially given its impossible to see what exactly the British would take from such a war. Hawaii? Cuba? Worth making a permanent enemy of the US next time the British had trouble in Europe?
