Why? The war's beginning was a very unlikely event, done without any sort of premeditation on the High Command in Tokyo. Even once it began, most people expected Chiang to cave and allow the formation of a larger Chinese puppet in NOrth China.
Some recent research has called the whole "Rouge Kwantung Army" into some question. Among other published works
Combined Fleet Decoded, where the authors are looking at original Japanese records, indicate that the entire series of events may have had a good deal more higher authority than previously believed.
Even if the Kwantung Army did go off on its own, the IJA didn't relieve commaners (much less encourage them to make amends to the Emperor) which would be the norm. Japan, overall, was attempting to fit an early 19th Century model of colonialism into a mid-20th Century event. Worse, it continued to push on in a bad situation (hardly an unique act, but particularly bad when you are contemplating a second, far more dangerous, war).
What about the Royal Navy being present in the region, or Australian troops?
The RN presence in the Pacific was far from debilitating to the British ( or Allied) War effort. Australian troops were deployed vs. the Germans in considerable number, with the number left being what the Australian government would have held for continental defense in any case.
The ETO followed a schedule that had little to nothing to do with the Pacific War. The bomber offensive was completely unaffected (at least until 1944 when the B-29 wasn't deployed to the ETO, by which time the combined bomber commands were blotting out the sun & stars over the Reich in any case), and the Battle of the Atlantic was decided by Intel more than vessels deployed. Ground combat, from Torch onward, were paced by available troops and landing craft construction. The USMC only had a total of two divisions deployed in 1942-43, with additional Army troops going into the Southwest Pacific in 1943 (when the manpower tap was opened sufficiently that it didn't cause problems in the ETO). The invasion of Sicily, Italy, even Normandy, happened at just about the earliest date possible for success. Especially in the case of Normandy any earlier attempt would have been an invitation to disaster.
Even if the Japanese hadn't gone to war with the West, the U.S. would have left force deployments for the USN virtually unchanged. The IJN presented a force in being and a possible threat, one that the U.S. had to honor.
Could the presence of several carriers, two or three BB's with the regular assocciated CA, CL, & DD escorts helped significantly in the Atlantic or Med starting in mid 1942?
It's hard to see how, considering the teething problems the USN had once it entered the war IOTL.
The Marines, which were the primary U.S. Pacific infantry force engaged in 1942, were also not well equipped to deal with ETO conditions. They were mostly a light infantry force, albeit one with more heavy equipment and far better developed combined arms doctrine than the IJA or IJN Landing Forces possessed. The Marines would have been good infantry, but the Allies HAD plenty of good infantry, and Marine formations generally lacked the logistical tail to engage in combat well inland (something that is still largely the case).
Could the Allies have taken Sicily a few days faster with 1st & 2nd Marines involved? Probably. Would that change the ETO in any significant way? No.
Would Italy have been changed into a rapid victory instead of a slugging match crawling across ground ideal for defense? Almost certainly not; although it is unlikely that a Marine led Anzio would have turned out as poorly as IOTL (of course it is hard to see how it could have gone worse).
Would Normandy have been easier with the troops engaged in the Pacific?
Very unlikely. There was only so much landing beach available & the thing the Allies needed most was better heavy armor; unfortunately, the Allies didn't HAVE heavy armor, nor did they realize that any was needed until after engaging the Heer.
The Pacific War didn't hurt the ETO, just as the famed "Europe First" strategy had minimal impact on the Pacific.