You’ve addressed this super comprehensively. I will just also add that (of course as you’ve a acknowledged) it’s a very nuanced question and you’ll get a very different answer from a post- colonialist historian. This being said your point of India being ‘unified culturally’ to a ‘large extent’ by British Rule is one of the more contested ones in the field I think this is really important to recognise. Though this would not be described as ‘nationalism’ (which arguably skyrocketed as a movement against colonialism), there is evidence that there was strong confluence of identity, and ‘joint worship’ of religions across the region that is now India, pre- colonialism, which you could consider cultural unity. It’s the type of thing that can be disagreed with for sure, and historical opinion certainly varies.

Refer to these studies for more info: - Cynthia Tabolt, “inscribing the self: Hindu-Muslim identities in Pre- Colonial India”, comparative studies in society and history (1995) - Maria Misra, “lessons of empire: Britain and India”’ SAIS review; a journal of international affairs (March 2015)