Well many circumstances can be created to overvcome Islamabad like
A host of Suicide Bombings followed by a flurry of militant activities.
A Lal Masjid like take and hold with larger numbers.
Or using a destroyer XXXXXXXI from space and zapping the city with alien killing beams.
Out of the three I think the third is more likely for the take over of Islamabad.
Thanks for confirming how impossible it is for Taliban to take over Islamabad any other way.