Japan will join the fight only when China is already about to lose. Read some Japanese defense type's interviews with the press: They want a divided and destroyed China more than anything; they cannot live in an Asia that is dominated by China again - their pride and their fear of facing up with their history prevents that; and will also join in stomping a China has is already lying on the ground, but they will hardly risk facing chinese wrath by joining early and losing alongside the others.
China must mass produce nuclear weapons immediately to deter the US from using nukes when China dominates the Taiwan war conventionally and sinks US carriers and strikes US bases. Nothing else matters right now. When the US has been dealt a bloody nose and is being kept from butthurt nuclear overreactions by Chinese nuclear deterrence, Japan, South Korea etc all will submit to the new world order.
I don't think it's in the interest of anyone to go overboard regarding the "China will defeat everyone" matter. But I've noticed that people usually see war in an one dimensional frame. A defeated China is a China gone rogue and insane. Even if under the remotest condition where China has to accept "defeat", the actions after that episode would be catastrophic for China domestically as well as internationally.
Domestically, political upheaval is going to be there. But since China is a great power, it can create a mess the likes of which will put the storied 'Samson Option' of Israel to shame. Ideally, for China's adversaries, they must create a condition for both domestic political upheaval in China as well as a Military defeat of China. A military defeat in itself, is not at all possible or desirable as people will rally hard during the trying times.
Somehow, they must link the military defeat to gross mismanagement ( very visible one at that) by the CCP of the PLA. But then if the war isn't over ( and China signing some document of concession of defeat) then the whole "responsible great power" tag will be torn away by China itself. A China signing a document of defeat is inconceivable in itself due to the sensitive past that China highlights to its children and citizen every other day.
What can China do as an "irresponsible" great power after that episode? A whole damn lot. In fact, I think it's scary for the world so interconnected through trade, Finance, data and resources. A great power is called so for a reason. No one will laugh a good laugh after a China fall.