I am most obliged.
My immediate thought on reading your posts was that the British had left a large number of loose ends at the time of their departure, a hasty departure, it has to be remembered. That was not just Mountbatten bringing the date forward to 1947 - it had been intended to be later - but also the total disarray in which many British officers from services, the Civil Service and the Police, the Forest Service, the Railways, and the Political Service, just to name a few, found themselves. They had planned on a tidy tenure and a well-deserved rest back home, in snug little bungalows; the days of the Nabobs were admittedly over, and nobody expected to retire to a Tudor mansion any more.
Perhaps due to the abruptness of their departure - perhaps - both the Dominions they left behind found a number of tangled knots on their borders. India had no end of trouble, continues to have trouble with her borders with China, formerly Tibet, currently the ruthlessly heeled province of Xijang. There is no doubt that there were anomalies; Aksai Chin was never a realistic claim, was in fact due to the cartographic privateering of renegade British geographers busy earning a dishonest coin from a gullible Maharaja. Tawang, too, was controversial, to say the least; it was a well-known appanage of Xigatse (Shigatse that was) and the entire territory had never been explored or penetrated other than by Bailley of Bailley's Trail fame.
That is where I see your problems with the Durand Line. That was exactly the same case as elsewhere, a line dictated without any understanding of local conditions, and imposed on a militarily disadvantaged opponent. It was immoral, some would say that, worse, it was impractical. The Durand Line should never have been imposed as a boundary between two integrated nation-states, as the British sought to present it, to nobody's conviction. Not even their own.
There is reason to sympathise with Afghan resentment of this line. The alternative to holding out on them on this point, however, was to give in completely. To jointly produce a boundary well within the Frontier Territory, and abandon everything west of that to the tribes that Afghanistan so bewilderingly embraced.
So should Pakistan cave in completely? No, not at all, or rather, not without gaining substantially from any arrangement. It is difficult to see a solution to this issue, for the simple reason that the ethos of those parts is strongly positioned on the tribe, not on the nation-state; sometimes, there seems to be room for doubt about the existence of the concept of the nation-state in the minds of the inhabitants of the Afghan-Pakistan borderlands. Can they be dominated and progressively converted to law-abiding, tolerant and peacefully engaged citizens?
The imagination boggles.