Was the Partition of India avoidable?
Of course it was!
One simple course of action to avoid Partition would have been for all the non-Muslims of India to voluntarily, peacefully and immediately convert to Islam. There would have been no Partition.
Of course, that is not what most people mean when they talk about this question. The correct question is -
"Was the Partition of India avoidable at a cost that was acceptable to the Indian nationalists?" (The Indian nationalists included the Indian National Congress, the Hindu Mahasabha, and a multitude of other parties, all of whom favored a united India. The main separatists were the All India Muslim League, and they had some minor company like the Communist Party of India, and some long-forgotten Dravidianists.)
"Was the Partition of India avoidable at a cost that was acceptable to the Indian nationalists?"
With the 20/20 vision of being able to look back at history, the answer to that question is -
No. The nuanced answer is -
most likely No.
Of course, no leader in 1946 could have known that answer with the certainty that we have today. And the nationalists did try very hard to avoid Partition.
So, e.g., when Tapan Ghosh @hstapanghosh tweets
For Partition Nehru alone was not responsible. It was the defeat/failure of our the then collective leadership & elite class Hindus society.
what was the defeat/failure of the collective leadership and elite-class Hindu society?
In my opinion, most of the leadership has still not grasped their failure. The failure was not in avoiding Partition; but in not anticipating and planning for the contingency that Partition might be unavoidable. B.R. Ambedkar is about the only leader who put some serious thought into that, and C. Rajagopalachari comes in a distant second in this arena. Everyone else, including Mahatma Gandhi, Jawaharlal Nehru, and Sardar Patel, were in deep denial. Many Indians today are still caught in the "those leaders could have avoided Partition, if only....". E.g., if only Jawaharlal Nehru hadn't had his press conference on July 10, 1946.
Once you accept the possibility that Partition might be unavoidable, you start thinking about how to get the best possible result for the Indian union that remains. We don't see much evidence of that happening except whatever little was possible in the very last couple of months of British rule.
Now, the British had their fingers on the scales, and it is not clear that any contingency planning to deal with an inevitable Partition would have been in any way fruitful. But there was no such thinking or planning, and even today, far too many Indians are still in the deep denial that I've mentioned above.
PS: The cost of accepting the Muslim League's demands that might have yielded a "united" India, here:
Parity and Communal Veto in Numbers.