So any talk of dividing India therefore is already on a wrong track because the question includes a false assumption of India constituting a single nation rather than actually constituting multiple nationalities?
That is a very interesting viewpoint. You are right that the underlying assumption does tend to go unchallenged. No way am I an India expert but I think it's definitely a viewpoint worth looking into.
Rather then a viewpoint, it is historical fact. It is actual history.
The largest ethnic group in China are the Han, they constitute more then 90% of China's population.
The largest ethnic group in South Asia, that's all the countries combined are the Bengali 13%, then Punjabi 9%, and many others with distinct cultures, history, language, foods, traditions and so on. A viewpoint is an interpretation of something, what I a stating is historical fact, facts as they have existed and as they exist now.
In all these centuries and millenniums of history why don't all the South Asians speak one language, Sanskrit is an ancient language so there was a reason to create a single identity, Hinduism was a one religion supposedly, so reasons for developing a single identity existed.
it did not happen because each group developed independently of each other, and co-existed purely by the force of geography, what else are you going to do except co-operate or fight with your neighbour, whilst defending and protecting your own identity, and that's exactly what has been happening. India was a region, never a country, a nation nor any single entity, that's a fact.
Even after 70 years of a modern state of India, India still does not have a national language, because they, the southern states, southern half of India and its Dravidian people refuse to accept Hindi as a national language. India has two official languages Hindi and English, and almost 2 dozen scheduled languages, They are both distinct categories, but no, I repeat no official language. How can they claim a historical nation when they cant even agree on a single national language after 70 years of independence.
There are other facts, but the above more then suffices to destroy their fantastical arguments.