I don't think Japan, Philippines and Korea will bend the knee just cause US is down. They are infected by extreme white worship and look down on China as inferior and not fit to be dominant.Everything you said doesn't matter. It's for an imaginary future but we are faced with a present challenge, which is America trying to drain the world's power into itself so it can fight China. If we don't beat this right now, we will have no future to plan for. So Always work against your enemy. If America wants to drain India, prevent it by protecting India. This is exactly what China is doing now because they agree with me and disagree with you. Chinese leaders understand to always work against your main enemy's and never get distracted by clown countries. India doesn't matter. Stronger or weaker, friendly or hostile, they are self-limiting. America matters. This is what we need to defeat RIGHT NOW. There is no future if you do not win NOW.
There is nothing special about Philippines, Japan, Vietnam. They are small countries that must foster good relations with the superpower in the area. You are missing the big picture because of what they do as American vassals. When America is down, they will all mellow and bend the knee to China. India is larger and has ambitions of its own but it's a future issue that cannot be prioritized over current issues.
Your problem is that you are so scared of a future Indian challenge that you are overlooking the present American challenge. You are so scared of India that you are about to bolster America, China's main enemy, instead. It's fear that blinds you to strategy.
The more US goes down, the more likely these countries will try to ally together, boost military budget and finally look develop nuclear weapons. They would rather become north Korea like pariah state than bend the knee to China.
I'm afraid China will have to fight a war, bomb one of these countries and perhaps foment a coup to be able to finally have dominant position. It will not be without violence.