Religion is the most potent method for interfering in any countries matter. 50% of Nagaland pop is Baptist inspired from the American variety. US maintains NGOs and religious organisation that fund and project.
NGOs can be dealt with but once u have made an impact using religious sentiment it is hard to counter.
The demand of Autonomy has one key to it and that is the claim that the state religion does not match the mainland and we are better off without them.
Now why would any Hindu want to separate from the only thing close to a Hindu country? It will always be a different religion for Islam its the various other Islamic countries that do Randi Rona and for Christianity it will be the entirety of the west that is hell bent on bringing Apocalypse 2.0 to gain salvation from this cursed earth.
There is only two things that can be done in Indias interest now in regards to NE.
1) This is kind Of impossible but regardless the best options. Convert them to Dharmic religion.
2) Instil nationalism in the Christian population which can't co exist with a Hindu majority, there will be tussle always.
You are right that religion is a best way to interfere in foreign countries matters.
But these tribes who decide to convert do it on the basis of tribalism.
These slit eyed savages all hate “Indians” and their pagan religion, so that is why they became Christian at the behest of superior “nice” white people.
Solution to this issue is to settle Nagaland with desis, look at Tripura and bongs, or Assamese.
It works very well.
As for the Nagas idc if they have to he butchered, driven away, or absorbed into desis via intermarriage or whatever.
The whole instill nationalistic sentiment is also a good idea, but why should they be nationalistic? They are alien most importantly in tribe and secondly in religion.
Which is why settle Nagaland with Hindu desis who should be patriotic and aggressive.
Works very well, Chinese , Americans , Israelis , Turkis of Turkey all have done it and continue to do it