Yes, its pretty easy to get to KP from Isb because its a left turn going up the hills, but the issue isnt solved by "ham idhar, tum udhar" typical myopia but by a combination of finding out Afghan and Afghan sympathizers and not picking up the random Pashtun just because you cannot see the issues created by past military operations and failed political cycles in the past.
And lol - Capital at Lahore - the same ones who were holding Javed Akhtar's feet as if it was the door to the Kaaba... Lahore is busy feeding itself and is best as the capital if you want Pakistani to capitulate within 2 days.
Islamabad was always a compromise as a site for a capital due to proximity to the overall Punjab biased communications lines and Ayub Khan's need to be closer to Pindi Garrison for his own safety and away from Karachi where Fatima Jinnah was always to be more popular.
It also offers "depth" of sorts from both sides of the border from some perspectives -
but if you want in ideal capital then think of Muzaffargarh as one option.