Did Jinnah ever regret his demand for Pakistan that triggered partition?
Yes, he did.
Based on the guide ‘Indian Summer: The History of the End of an Empire’ written by a British historian Alex von Tunzelmann, Jinnah called the interest in Pakistan the biggest error of his life.
Jinnah, who suffered from pneumonia, lung cancer, and tuberculosis, was to Karachi over a trip from Quetta when in a situation of delirium he told his sister Fatima that:
Kashmir… present soon…Refugees…give them…all assistance…Pakistan… will be complete…ed by them…the right…to decide…Constitution…I
He saw the Prime Minister Liaquat Ali Khan and told him that ‘the demand for Pakistan was something else along the lines of and my lifen's greatest oversight on reaching Karachi airport:
If now I get a chance I will goto Delhi and inform Jawaharlal to neglect the follies of the past and start to become friends.
He was subsequently taken from the airport within an ambulance towards the Governor General's residence. He whispered to his cousin Fatima, later that night:
Fati, khuda Hafiz. La ilaha…illallah…Muhammadur …rasulullah.
That is considered among the fundamental tenets of Islam. These terms coming from the lips of Jinnah, who lived nearly his life time as an atheist, in many cases are taken as a confession of his faith.
A couple of hours later that evening, Jinnah died.
Today, since Jinnah was quite sensitive in his nights, both physically and psychologically, it's very difficult to determine just how much he might have believed in these minute confessions. Was he a thinking Muslim at heart? Did he truly regret demanding Pakistan which causes the horrible effects of partition? Or were these uttered in a disordered state of mind, just feverish ramblings of the ill old man thus shouldn't be studied seriously?
One can not be sure.
However, it could be explained with some guarantee the words of regret concerning the partition did come over the past times of his life out of his mouth.