Looked up the old British surveys. 75% Muslim in the state as a whole from late 1800s right up to just before Independence. I presume you will agree the British had no particular ax to grind in their census.
I am aware of terrorism against Hindus, but that didn’t create the Muslim majority.
Actually they did, especially in the mid-to-late 19th century - the British favoured the Muslims over the Hindus then - dealing with the Mughals and their vassals. It took the 1857 revolt to change that.
The British had plenty of ax to grind. I cannot comment on the authenticity of the census you are taking about and what regions it covered. I have a feeling it probably does not include Ladakh in the census.
Either ways, Muslims majority does not default to Pakistani ownership because India isn't a Hindu country. India has more Muslims then Pakistan. And secondly a large population of Kashmiri Muslims are Shia as opposed to majority of Pakistani population being Sunni dominated. Shias are firmly with India. And lastly a large population of Sunni Muslims in the valley are with India because they loath the Pakistani Punjabi majority ruling class that has demographically displaced ethnic Kashmiris in the part of Kashmir under Pakistani control. The Kashmiris on the other side of the border only serve as endless supply of jihadi foot soldiers to carry on Pakistan's low intensity war against India, which passes for "Kashmiri freedom struggle against Indian rule".