At this point, China has more or less backed off. They've ensured that their rival, India, has a major threat to contend with on their border. They don't need to keep their hands in that pot anymore. Pakistan's as dangerous as they're going to get.
There's a number of delivery methods for nuclear weapons, especially if you're open to a sneak attack. I wouldn't pin too much hope on counter-missile systems just yet. It's a smart development, but no magic shield.
As far as sterilizing the country, Pakistan is about five nuclear weapon strikes from being reduced to a bronze age status. Take out the major population centers, and the country will disintegrate. Why spend expensive nuclear weapons to make the rubble bounce?
“Why spend expensive nuclear weapons to make the rubble bounce?”
I’m a big fan of neutron bombs. This uses the weird logic that they don’t care as much about their lives than they do their “stuff”. If every lifeform in Pakistan would just die, it would likely mean that India would get their stuff. And the very thought of their hated enemy getting their stuff is enough to drive them mad.