Of course missiles can fly both ways. Attack Iraq, and there'd be incoming from at least the US, and possibly Britain. Attack Israel, and before the warheads went off, there would be many more headed back (and since Israel has an anti IRBM system, many Pak warheads would not make it).
Of course the Mullahs are Mad, and they might not care much if there targets destroyed Pakistan and probably Iran as well. As long as the net result was a furtherance of Global Jihad, and their butts were same in some deep cave, they'd probably not give a flip.
Then there is the "sneak nukes into US cities, bypassing luanch systems and all that, and lending some uncertainly as to the source of the nukes.)