India would have many more grouping than religious divides : 1. Pro Hindustani/Devanagari/Urdu languages (majority of Indus-Ganges river area) vs Pro English (most peripheral region) 2. Pro Central Government (business class, college graduates, modernist, groups who didn't own states) vs Pro States groups 3. Urban vs Rural 4. Rich vs Middle-class vs Poor 5. Religious Poor vs Socialist Poor
So Religious divide wouldn't be all-important. The Big Decider would be fate of "muhajirs" classes who are middle-class Muslims living in Hindu cities and states, If these people stay rich, idea if Islamic separatism would evaporate. Democrats/Populist would fight them bitterly (not just Hindu, but also Bengal and Sindhi) for control of government and economy. If Feds could protect them and help modernize India, they (together with Hindu and Others city-based middle class) might be backbone for federal government support.
United India would also had "interesting" problem concerning Baloch (spread between Iran and India), Tibetan (China-India), Pashtun (Afghanistan-India), Bengals (Burma-India), and tribesmen in regions shared by Burma-India. many of these groups would have their own states in India, with support for language and culture, and may help their co-ethnic across border.
Weirdly, Pashtun was big on Socialist before partition.