Some correction's my friend,on record Tibet had accepted that Arunachal belong to India and India had full controll over it,so ur point fall flat over the matter,why China should have it claim over Arunachal.
Here u have an another misconception that India is all about hinduism,I am not bringing points on this topic because it will be way off topic and will attract some unwanted attraction from religious trolls,anyway even just like what u assume,arunachal have its connection with hinduism,such as the 14th century Malinithan temple at the foot of the Siang hills(which precedes ur Quing dynasty by a good time).
Do u really believe leaving the territory by China was a honest gesture,seems more accountability was in ur words before that,actualy to avoid further conflict.
Avoiding further conflict is not a nice gesture even after we completely crashed your army... even after Nehru wanted to evacuate Delhi???
Do you think then Chairman Mao is so scared of India retaliation so that we withdrew?
If that was the case, there should not be any Korean war in the 50s or Border Conflict with USSR over Zhenbao (treasure) Island in the Northeast in the late 60s? There won't be hundreds of thousands of Chinese Army into Vietnam to help Northern Vietnam to fight the US during the 60s either.
The most important reason is that China does not want to have a complete breakdown of relationship with India since both are important Third World Country and Non-Alliance movement. Mao did not want to be taken advantage by his supposed imperialist countries like US and the west.