However, if the Commonwealth had NOT been busy in Europe, there would have been no such power vacuum, and this Pacific war would not have started.
I support your general ideas here but disagree with this one.
Japan did not intend to start a war with their attacks on the Commonwealth and the Americans. They truly believed that a decisive attack would break the spirit of the Commonwealth and the Americans and this belief came about through their culture of Bushido in which a warrior would be defeated and then shamed into no longer fighting. It was reinforced with the Japanese victory in the Russo-Japanese War and if you read the Wiki page you'll see the cultural setting that the war was set against for the Japanese.
https://en.wikipedia.org/wiki/Russo-Japanese_War
The outcome of that war reinforced to the Japanese their version of American
Manifest Destiny and they truly believed their actions were divinely inspired. They also believed they had a divine right to rule the Western Pacific.
So while a few Japanese military leaders understood that Japan was starting a war they could not win the majority of Japanese and their leaders saw the attack on Pearl Harbor and the conquest of European holdings in the Pacific as limited actions that were intended to show the West that Japan was now the boss in the Pacific.
The Japanese and the Germans also shared a deep misunderstanding of the American manufacturing industry. They thought it was wasteful that American companies would retool their factories every year to produce new cars and new products for the market. Meanwhile German and Japanese factories rarely retooled and would produce the same products for years and years.
The Japanese and Germans mocked the Americans for manufacturing washing machines, toy Erector sets, and for making a new model car every single year. They saw this as a weakness and a character flaw that Americans prized consumer goods over weapons.
Big mistake.
Once the Japanese attack had erased most of the isolationist and pacifist sentiments in the USA the consumer manufacturing industry went to war.
The world's largest auto factory in Chicago was the Dodge plant. Dodge put their expertise in retooling to work and as they produced their last 1941 automobile they tore up the assembly line behind it and retooled to build B-24 bombers. A mere seven days after the last car was produced the Dodge plant produced their first B-24 bomber.
Maytag washing machine company took their washing machine expertise and put it to work making turrets for aircraft and for anti-aircraft guns.
The Erector Set toy company retooled and started making Thompson submachine guns for the war effort.
In the space of a few weeks many key industries started war production. The rest of the nation followed and by the end of the war the US industrial base was literally producing more aircraft, bombs, weapons, and supplies than anyone could even use.
Japan and Germany drowned in American war materiel.
Their biases and their mistaken belief in their own superiority made them think they could strike a decisive blow against America and that the soft Americans would crumble. They also thought that the warships they destroyed at Pearl Harbor and elsewhere would take years and years to be replaced. Most of them only took a few months to be repaired and put back into service and the Japanese and Germans failed to grasp the potential of the American salvage effort that employed techniques obtained from building railroads and bridges.
In short, absent a war in Europe I believe the Japanese would have done the same things simply because their over-confidence and their misconceptions of the West and the American people would have led them to do these things anyway.