In the eastern sector, however, the entire disputed territory hinges upon one question - The legality, or not, of the Simla agreement.
India has had two contradictory stances simultaneously
a) Not recognizing Tibet's sovereignty and
b) Recognizing the McMahon line as the international boundary;
and thus the legality of the Simla agreement. However, if a country doesn't recognize Tibet's sovereignty, then consequently it is expected that it would also not recognize the legality of the Simla agreement and the McMahon line.
The Indian position can also be construed to mean that regardless of whether or not Tibet is sovereign now , it was sovereign when the Simla agreement was signed; and consequently the McMahon line is legal.
If India recognized Tibet sovereign which meant India illegally invaded and occupied Tibetans land. The more Indian open their mouth about illegal occupation of Tibet by China, the more they stuck their foot in the mouth.
For McMahon line to be a legal document, first thing India should do is to recognize Tibet as a sovereign state, and backtrack it to at least 1914. Not doing so, and neither did British India did so, make the Shimla Accord 1914 a piece of rubbish and that is the reason China found it unacceptable.