Six point was a demand that could not be achieved by the Pakistan Awami League in the Pakistan National Assembly convened on 25th March, 1971. A 2/3rd majority is needed to revise, change or rectify even a colon or semicolon of the Constitution. PAL had only a simple majority and there was no way it could have passed any changes without support from Members from the west. Wali Khan NAP was also in favour of a kind of autonomy for the NWFP. But, there was no strategic alliance between his party and PAL.
It can be assumed that PAL would not have been able to get any big concessions in the NA. But, Yahya Khan wanted a showdown only to teach the Bangalis a lesson. In that way he wanted to cling to power for a few months, too. Note also that Z.A. Bhutto wanted to be the FM of Pakistan. He became the PM by betraying democracy and dividing the country. Mujib was the last man who wanted to break Pakistan. He had no such plan. Otherwise, he would have hided among the millions and participated in the Liberation War.
PAL was an inefficient organization without any experience to run the administration of a country. This inept AL could have been ousted any time after a 3 month period. PA was always very strong. It could have arranged another coup at a later time. But, PA chose to break the country into pieces instead of waiting for another opportunity. Moreover, PAL wanted Yahya Khan to remain President for an indefinite period. It is the loss of patience by stoneheaded west Pakistan politicians that caused Pakistan's division.