I think you are too simplifying things between Bengali, Punjabi, Pathan or Sindhi, Baluch. The issue was the 6-point demand by the PAL party. This is what ZA Bhutto did not want to give because it would have caused more bifurcation in the west. This is what he and other politicians belonging to Muslim League and other parties thought at that time.Basically Bhutto was not ready to accept a Bengali being the head of the govt. in Pakistan for some reason and wanted it all for himself. I still don't get this "idhar hum, udhar tum" business....
Perhaps, you are not aware that Khwaja Nazimuddin became the 2nd Governer General of Pakistan after the death of Quaid-e-Azam. He again became the PM of Pakistan.
Iskandar Mirza became also the President of Pakistan. He belonged to the House of Mir Zafar Ali Khan, the Nawab of Bengal.
In between, there are Muhammed Ali Bogra and Hussein Shahid Suhrawardy who became also the PM.
After east Pakistan seceded, Nurul Amin became also the PM of Pakistan.