I would build missile defences in Pakistan, Bangladesh and Sri Lanka and station Chinese air and naval assets in military bases in those countries.
China needs not to worry, India is already claiming that china is building a string of pearls with deep sea port development in each of the countries you mentioned.
Here's the run down.
China has helped build the
Gwadar port in Pakistan
Colombo International Container Terminal/Port in Sri Lanka
Planning to help build ports at in/
A deep sea port at Sonadia Island, Cox's Bazaar in Bangladesh
Also an ongoing feasability study to build a deep sea port in Kutubdia island, Chittagong in Bangladesh
A dry port in Nepal.
China is also looking at Myanmar.
All the countries you listed said that they will not allow any country to operate militarily on their dea ports.
China has also planned to help build several townships in the countries mentioned above.
China's primary country of Focus in South Asia is Bangladesh. This is because Bangladesh is located at the mouth of the Bay of Bengal, that would mean they have control over a majority of the Bay of Bengal and the indian ocean, in terms of trade, commerce and an advantage in warfare. Bangladesh is also the gateway from south asia to south east asia, vice versa, meaning that it can potentially be a stopping point for goods as far as hong kong traveling to Mumbai, by road.
Infact china isn't the only country interested in Bangladesh, so is America and Russia, both are interested for the first point I made, if you have a base in the bay of bengal, than you control major sea routes.
What's on the line?
Regional connectivity which boast the india's seven sister states, Bangladesh and China are on the line. The BCIM corridor as I mentioned before, is a vital trade route, which dates back to silk road era and is being proposed again, for regional connectivity, if a conflict occurs than the BCIM corridor isn't likely to happen.
Also Both China and India are probably going to be in a losing battle. If a conflict breaks out between China and India, than spratist movement in both countries will probably engage in war too, and try and succed from one another?
A run down of hostile areas in Both countries.
China: Tibet Autonomous Region,, Xijiang province, Inner Mongolia and Hong Kong.
India, all the seven siter states, Arunachal Pradesh,Assam, Manipur, Meghalaya, Mizoram, Nagaland, and Tripura, Jammu and Kashmir.
Either way it won't look good for any of the countries. Both countries will be wedged in conflict between each other and conflict inside their own country.
India should just back the f*** up, but they probably wont cause they still alldge that china is building a string of pearls in south asia, that's probably why India has keen interest in viet nam, cause than it can get back at china.
Also I would like to note that if the bay of bengal/Bangladesh engages in war than Japan would be there to help it due to a defence contract signed earlier by the two armies. Bangladesh and Japan and Malaysia are on good terms.