Your assessment is wrong, if China divert water from Brahmaputra river, not only India will get nothing from China and but if China divert excessively the water it will be come a reversed flown which mean China will get water from India
and drastically lower the level of water of Brahmaputra and you can kiss good bye of your North East. Your people need this water to survive more than us but if this water can bring back the reforestation of Xinjiang...then it's worth for China to carry on.
The reason China still doing nothing about Tibet water is because China hope to have India as friend and not to piss it off and make India obsessed over the water issue, but when become enemy China will not be polite with India when come to national interest.