Japan will for sure join because it's an existentialist question for present day Japan (at least politically). SK will join due to US control of the CFC.
Philippines and India is hard to predict. I think the Philippines aids in minor ways - with the US being unsatisfied with their involvement. India will only join if China starts to show weakness. A second front in the west is a death sentence for any Chinese military conflict and America and India recognizes that. India won't dare start something if China looks strong, but if China looks weak they will move swiftly into war. Then again Pakistan might get involved too, which may cause India to hesitate.
Singapore and Indonesia won't join at all. Singapore will sit on the sidelines until a winner becomes clear. Indonesia will take this opportunity to gain influence by remaining neutral.
Imo it will be China vs USA/Japan/SK/Australia/UK. India and Philippines dipping their toes in to help. Everyone else stays neutral.
If SK has declared war on China, there is no way that NK will be neutral.
And if South Korea is neutral and China is losing, NK will not be neutral either.
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The reason is that NK is economically dependent on China, which accounts for 90% of NK trade. NK face the prospect of economic collapse and being next on the list of regime change. NK can't afford to have China lose.
And in a land war on the Korean Peninsula being CN/NK versus SK/US, it looks like Ukraine but with some important differences that favour China, eg.
1. SK is essentially an island which will likely be blockaded
2. Seoul accounts for half the SK population and is on the East Coast opposite, close by China
2. a land war favours the Chinese Military over an air-sea war
3. Equipment-wise, China is likely to really outproduce the opposition in terms of ground equipment and munitions
