While China itself has little to worry about in terms of a potential US invasion of the mainland (I'm pretty sure the US has no interest in attempting this particular scenario), the US also doesn't actually need its allies to "hold out" for any length of time, as if China actually had the ability to take any of its surrounding countries, excepting possibly Taiwan. Japan, South Korea, the Philippines, and Australia all have little to worry about in terms of an invasion from China. So really what are we talking about here? A US invasion of the Chinese mainland? Extremely unlikely. A Chinese invasion of any of its US-allied neighbors? Extremely unlikely. A Taiwan invasion scenario? This is the only scenario which is even remotely a possibility for a US-China conflict.
Well, little wonder the rest of your post makes so little sense - you have fundemantally misunderstood the nature of the threat and question.
The examples you listed above will never be made at the local field or regional command levels. Those are way above the pay grade of even the commander of US Pacific forces.
Any Chinese invasion of an US treaty ally will trigger the mutual defence treaties in place, in which case the regional commander has minimal strategic choice. Barring a direct countermand from PONTUS, those kinds of examples will fall firmly on the list of exceptional circumstances I mentioned, where US forces will be engaging out of necessity rather than choice, and will be expending lives in large numbers to buy the time it will need for reinforcements to arrive.
As you rightly noted, such examples are exceedingly unlikely, and other than knowing that there are dedicated eggheads gaming out scenarios and writing a playbook they could consult in the event anything like that actually happened, I seriously doubt any US field commander is going to be devoting much, if any of their time and attention to thinking about such scenarios.
The real scenarios that keeps such field commanders up at night, and what no doubt prompted the original remark, are the unintended clashes that could arise out of the blue to trigger a full scale conflict.
These events could trigger a vicious, but short and contained limited war, which will be over before the US could deploy enough reinforcements to make a decisive difference.
An example would be a clash between Chinese and Japanese patrol ships around the Diaoyu islands.
Civilian coast guard ships face off against each other; either through error in judgement, an accident, strong emotions or a combination of all those factors, there is a collusion between the two ships and the Japanese ship sinks.
Another nearby Japanese coast guard ship opens fire and sinks the unarmed Chinese coast guard ship.
A nearby PLAN warship opens fire and sinks the Japanese coast guard ship.
A nearby Japanese warship opens fire on the Chinese warship, so on and so forth.
Same deal with a clash in the air in China's ADIZ.
Chinese fighters scramble to intercept Japanese fighter. Both sides engage in a little 'aggressive manoeuvring'. One fighter clips another, causing it to break up with no chute, wingman of lost fighter gets emotional and opens fire.
Different trigger, same outcome.
And those are just the obvious examples of the top of my head. If I could be bothered to, I'm sure I can dream up many others.
It's actually frighteningly easy to see how such a small spark could trigger a full blown limited war between China and Japan.
All sides will know that this was an unintentional conflict, and no side wants a full-scale war on their hands. So all sides wants to end it as soon as possible, yet they will also want to emerge as the clear victor.
Because China knows it enjoys initial local tactical and strategic advantages, which will start to erode more and more the longer the conflict drags on for as the US redeploy a forces from elsewhere, China will have the clear incentive to move as rapidly up the escalation ladder as possible (within reason of course before you start), with the aim of delivering a knock-out blow to win the fight before the US can risk direct intervention.
China could then 'magnanimously' call a unilateral ceasefire, with a full spectrum diplomatic offensive to stress that Chinese forces were only defending themselves, and are proving that by only holding ground instead of advancing to launch follow-on attacks against the now 'defenceless' Japanese as a show of 'good faith'.
That would make it exceptionally difficult for the US to re-initiate hostilities after it had gathered enough forces to risk it.
That is going to be the Chinese play, and the commander of US forces will know that, so is going to have a tough choice to make - either to leave the Japanese to get beaten up by the Chinese while the USN sits the whole thing out, and risk fundamentally undermining the value of US defence commitments; or be forced to send the forces under his command into the fray without waiting for reinforcements, against an enemy that will enjoy local tactical and strategic military advantages even with the might of his forces added to those of the Japanese, so runs the risk that China will be able to defeat both sets of forces and still call its unilateral ceasefire to hamstring the US follow-up response.
If that were to happen, it will still be exceptionally hard diplomatically and politically for the US to re-open hostilities if it was the US forces who opened fire on the Chinese first.
That is the 'nightmare' scenario that would have been playing on the back of the mind of the Admiral when he stated that the US isn't the biggest guy on the block - it isn't, and that matters.
The local disposition of forces, and opponent faced absolutely plays a critical part in the decision making process of US field commanders.
Remember those recent news clips of Russian fighters overflying USN warships? Do you really think the USN would have allowed the fighters to get that close if they were Iranian?
Hell no, if those fighters were Iranian, the USN would have "lit 'em up" as a warning long before they got within visual range of the ship, and shot them down before allowing them to overfly the ship.
The ROEs are different because in the case of Iran, the USN knows it has overwhelming local force superiority to win any conflict scenario, so has little fear of Iranian retaliation.
Against the likes of China and Russia, local US field commanders will have far stricter ROEs and engagement thresholds when it comes to actively initiate hostilities in support of allies. That is the reality of not being the biggest guy on the block.
Your Taiwan scenario is both entirely off topic, and shows some considerable bias and deviation from reality. It almost feels like a deliberate downplaying of Chinese military capabilities as retribution for some imaginary slight against US military prowess.
Either that or your judgement is well and truly suspect if you honestly believe that fantasy.
Regardless, I see nothing productive coming from any more wasted words on that red herring.
The admiral stating that the US is no longer the biggest guy on the block is facetious at best. You can try and weasel it in that he was talking about forward deployed forces if you want, but even if he was this distinction as it stands now is essentially meaningless.
'Weasel'? Getting a bit personal are we not? Did I bruise your ego with facts?
Uhh, no. Sadly, your giant wall of verbiage about the SCS...
Yet more personal attacks. Classy.
Once again, your deliberate downplaying of Chinese capabilities is disappointing.