Admittedly your assessment is fairly accurate. With the balance of firepower slowly shifting towards chinas favor predictive model anlaysis and scenario done by certain organizations indicate outcomes on similar nature. It’s no state secret and relatively common knowledge.
In a conventional and limited conflict scenario, the US 7th with JMSDF would have a difficult time containaing both PLAN and PLAAF due mainly to logistical issues. Even with 2 CSGs, an entire wing of Raptors and all of JMSDF brought to bear, the balance of capabilities would still be extremely strained. Patriot batteries can only do so much in massive saturation attacks and even then tested only in simulation never in real life. SK and Australia like you said would sit it out unless china is the clear aggressor or other geopolitical factors at play.
The only way to win is to never play in the first place.
Sometimes you do not get to choose...sometomes your are forced to play.
THiose who believe that South Korea would stand by while the US and JApan fought for their lives are, IMHO, sorely underestimating the SKOR mind set.
They are and have been solid allies for decades...and I believe if it came down to it, they would stanf by that alliance.
Like Kwaig, I pray we never have to find out...but as I say, and as we have found in the past...somtimes you are not left with a choice.
As it is, US, JMSDF, ROKN, and the RAN together are a very powerfu...and for the forseeable future, an allied force that is not about to be overpowered at this stage.
With current projections, particularly if the US continues to turn things around...I do not believe it would ever become wise for either side to test that.
But, others have, in the past, done so. I pray that it does not occur again...but I also pray that we will not forget the hard lessons that that type of peace is best preserved through strength.
Time will tell...and I really expect, with my own health, and also be cause I honestly believe God in Heaven has other plans...that it does not happen at all.