Believe it or not, I AGREE with some of the possibilities you proposed. If a predisposed individual terrorist acquired a nuclear device and detonated it in a large metropolitan city, anywhere in the world, the result would be monumental shock!
The world would in fact stand still again, as all media stops and focuses all of its attention on yet another disaster. All available radio and tv would broadcast reports and updates 24/7, like they did on 9/11.
I guess the question is, would the governments of this world allow a retaliation of similar proportions? Who or what organization would/should be the target? Once a massive amount of "man-made" destruction has occured, the tendency of most humans is to prevent a reoccurrance at all costs.
In order to punish "a" terrorist, or "terrorists", would anyone permit the senseless destruction of hundreds of thousands of other innocent lives? As long as a terrorist can "mingle" with the population, he is safe. As long as criminals, terrorists, warlords, or mercenaries mingle with people, they are safe from such action.
Now, as you purport, suppose a government (INDIA/PAKISTAN) launches a nuclear weapon? Would the rest of the world "plunge" right in and escalate? I don't honestly believe that for a minute. The world would in fact "slam" the brakes on immediately.
People desire life and happiness. Now, I caution "bible-thumpers" of all kinds because you tend to exhibit the same "hysteria" occultists display. Occultists, astrologists, card-readers, mediums, and their followers are "addicted" to SECRET KNOWLEDGE OF THE FUTUE.
So are religionists. The occultists believe magic and the supernatural are the gateways to this knowledge and religionists rely on interpreting "signs" through the bible and world events. Both are addictions when measured scientifically. The dangers of these addictions are "false sense of reaity","delusion","paranoia","nervous anxieties","depression" and a host of others. In addition to the physical dangers, are the economic and social dangers of "living" in another dimension, while time continues moving forward.
It certainly cannot be physcically, emotionally, socially, economically healthy to be preoccupied with "doom, gloom and disaster", can it? Perhaps, if you convert (no pun intended) your preoccupation into a business, you might become rich from the curiosity and devotion of your peers. Then, it might make sense in a "hidden" way. "John Edwards" of "Crossing Over" is a current example of such well paid buffoonery.
Fear is an improper use of the imaginaiton. We all fear of death and the unknown. However, to allow fear to become our dominant emotion, preoccupation would make us vulnerable to predators.