>There will never be a hot war between US and China for so long as nuclear weapons have no effective defense.
I beg to differ. First, see India v. Pakistan as a direct counter to your "impossible" statement.
Second, as the example shows, the line you draw regarding using nukes does not extend all the way down to the rifleman. It's entirely possible to start a conventional and/or localized war with the understanding that you intend to win and therefore do not have the need to use your nukes.
You're making the major mistake though of assuming your enemy is a rational actor, a la the cold war. Russia is a rational actor. So is China. Even India.
In Pakistan however, only a minority are rational actors. Should a change in government occur that is pro-Taliban, you now have a nation of irrational actors with nuclear weapons.
MAD falls apart at this point. The fear of death means little to the Muslim extremist, and a man that isn't scared does highly irrational things, like nuke a civilian target.
All it takes is one, and then all bets are off.