And what is understood by one nation ?
Like Pakistan and Bangladesh were one nation ?
There are significant differences between the case of East Pakistan and the Pakhtun's in Pakistan - geographic separation, lack of representation in accordance with the size of Bengali population etc. The most significant factors behind the unrest in East Pakistan are absent in the case of the Pakistani Pashtun.
Nationhood in the subcontinent is and has been very diffrent kind of a bird rather than the European bird. Say for example Punjab remained Punjab whether it was under Kaniska, Akbar, British or now parts of India and Pakistan. Similarily, the tribes always remained historically undevided or devided insite of being parts of Afghanistan and Pakistan. It will be difficult for any one to force them into Pakistani type of Nationalism. The cultural nationalism of Pashtoons if turned into a political force will be akin to awakening Durranies who will rule over Pakistan.
You are conflating two very similar yet not quite analogous concepts - 'Pakistani nationalism' vs 'Uniting under the flag of Pakistan'.
Nationalism/patriotism comes with time, with integration into a society/nation, with a sense of 'belonging' etc.
Unification under the flag of a single nation can be (not guaranteed by any means) prequel to nationalism/patriotism. FATA is an example of 'unification under the flag of Pakistan', and while a degree of 'integration into Pakistan' has been accomplished (note the failure of the Afghan's historically to successfully incite the tribes into revolting against Pakistan and joining Afghanistan), the process is by no means complete. What the example of FATA illustrates is that it is possible to 'unite disparate and autonomous tribal groups under the flag of one nation' - where FATA is a failure is in terms of a subsequent comprehensive integration of said tribal groups into the political, economic and administrative mainstream of the country.
This is obviously a long term process, and the FATA tribes have indeed built linkages across Pakistan, from Rawalpindi to Quetta to Karachi, but those links affect to small a number and are not part of any sustained government driven process to integrate FATA into the rest of the country, politically or economically.
Pakistan is treading on a dangerious path. If Pathans become one, Pakistaniyat will be lost !
What is interesting is that the FATA tribes, from what I know, are quite divided about the idea of being integrated into the Pakistani province of Khyber-Pakhtunkhwa - many in fact support the idea of a separate province, or more. The KP Hazara community (Hindko speaking residents of KP, not to be confused with the ethnic Hazara of Balochistan and Afghanistan) are also demanding a separate province carved out of KP. Of course the ANP adamantly opposes these demands but the demands highlight the differences and divergent interests of the Pakistani Pashtun community,