Nowhere did I really mention the 'wrong boundaries' issue, I personally think that that case is overstated.
India was partitioned into two states (Pakistan and India, no Bangladesh at that time) but the Republic of India has faced no serious problems of ethnic unrest despite its multi-ethnic nature, and many languages. Pakistan's problems by contrast stem in large part from the fact that people there felt the whole show was being run by the Punjabis for the benefit of the Punjabis, which is how they lost East Pakistan
India and Pakistan (including East Pakistan) shared so little in common with India that they created one of the worst humanitarian catastrophes in the modern era as India broke up. East and West Pakistan again had little in common that this collapsed. Punjabi vs Bengali issues implied that there was a) no common Indian identity b) even after partition, the common Pakistani identity was very weak.
I'm not sure how Kashmir isn't a "serious problem of unrest". It's been an issue for decades and India and Pakistan have fought wars over it.
Indians are Indians in the sense that Britons are British. They have sub-nations below this identity that are every bit as important and could replace the Indian identity in the right conditions.
India is not a (qualified) success because it had a strong united identity. It is a (qualified) success despite having very diverse identities.
Somalia isn't a failure because of a lack of a cohesive national identity. It is a failure despite having a cohesive national identity.