4th in a series of Muslim-wanks to try to bring some balance when Muslim-screws are vastly more popular on the board:
AHC: How can Islam be far more successful than in OTL in converting the population of India? In such a circumstances where many of OTL's current Hindu-majority states become majority Muslim instead, what geographic areas of India would be the most likely final holdouts of Hinduism?
Well, to begin with, let us assume you mean the entirety of the Indian subcontinent, in which case Muslims constitute 35% of the continent. This is for purposes of calculation and history.
The first proper interaction with Islam would be the Ghazni raids, which alienated quite a few Hindus and other religions too, and to this day is a source of resentment. That would have to be undone. Instead, the possibility of a Ghurite invasion being the first taste of Islam that India has would be much more beneficial. Now, assuming that Qutub-Din Aibak is the same as he is in OTL and known as a funder of temples, there is not much threat of either Sultanate-wide rebellion or alienation with the Hindu populace. Now, coming to the creation of the Islamic populace in India. Unlike other Islamic communes around the world, the Islam practiced in India inexplicably had its own caste boundaries, which were considered by them to be independent of religion. This backwardness was a major failure of Islam in India, as rather than aiming to be a liberator from shackles of caste oppression, it integrated itself into an Indian setting. Islam probably would have more followers in India if it was aimed at the lower castes.
Another thing that could have increase India's Islamic population was the lack of divisive British propaganda. The British often aimed at creating divided represantations of Muslim and Hindu rule, and ended up creating a chain of events that ultimately led to Partition. However, at a more present time, if the British hadn't totally annihiliated Muslim potentates in India, through annexation or subsidiary alliances, never mind that, there probably would be a gradual change to Islam.
To answer the last part of your question, India doesn't work on a religious-ethnic basic, but on a basis in which the two are independent of each other. Hindus and Muslims would probably be dispersed throughout the country.