2: Tactical (not strategic) nuclear weapons are very likely to be used on the battlefield in the next major conflict involving the superpowers. The most plausible pathway to a nuclear armageddon is if the number and yield of these tactical weapons keeps escalating until one side finally has enough and launches a strategic strike.
I'm not sure that's entirely true. Even if DC gets nuked before it can react, I'm pretty sure there are dead man switch-type systems in place and stuff like submarines equipped with nukes away from home so that even if they whole country was glassed there would be some retaliation. These scenarios probably don't reflect the whole arsenal, so the retaliation wouldn't hit every square inch of the enemy's land, but I would still think that threat is enough to make you not want to start anything.
Check.
Imagine China fired all her nukes at the US and the US retaliated while the Chinese ones were in the air... there would be nothing left of either country within a couple of hours (or however long it takes to reach across the globe).
Anything left would be uninhabitable forever.
I dunno... nukes are messy.
E-warfare is "clean" and costs practically nothing to do.