Well I think the likelyhood of Japan taking both China and Sinkiang is very low in fact. If you think about it Japan starts with only 7inf on the ground and 3ftrs bmb for offense. China with 2inf ftr is more dangerous than one would expect and because of this it is my feeling that you must send at least 3inf ftr bmb just to guarantee a good shot at clearing it. Additionally, Sink can only be attacked with 2inf + some air and is a lesser priority so in fact it is safe to assume that one of these will go badly. Further Japan spreading out its offensive ability on turn 1 is a high risk high reward move as you could also lose all these battles which will really slow down Japan. Losing say 6inf just to kill an irrelevent 2inf ftr or an extra 2inf is not a good idea. Further, if the Brits pull just 1 Indian guy or 1 Syrian back to Persia they will have an opportunity to attack the infantry put in India by Japan. If the Brits can nibble at these forces while spread out, and the Japanese hurt themselves by spreading out then it is far easier to slow Japan down than if you play defense where Japan will maintain its relative strength by virtue of not attacking.
Secondly, what gain is there to moving 2inf from India to Sink anyway? If the Japanese want to risk it they can still attack it, and if they do you risk losing more of what little strength in Asia you do have. Additionally, Japan can still take India, and Britain is still left attacking with 1inf + air to clear it so what’s the difference?
Finally, your primary reason against abandoning India is that the IPC swing is too great but in fact even with a defense of India or Sink Japan can still attack one or the other, and for that reason the IPC swing is almost as great so in fact the vacating of India actually costs the Brits very little. Whether the British are in India or not the Japanese should be able to take China and India or Sink which is a 10ipcs swing. The difference is but only 1guy which is more than worth it for the British.