No, but it would be alright, because those governments are not the terrorists themselves. Russia could us diplomatic or conventional military forces to deal with the terrorists. It Iran started bombing the US then sure why not? But I made a point of combat against the USSR, terrorism, spying, even encouraging a coup is not military combat.
What my scenario does do is discourage conventional military, and nuclear attacks against Russia. If Chenyain terrorists attacks Russia it will not end in nuclear war, because that isn't a government, its a group.
If Russia is reduced to using conventional means to respond to a nuclear terrorist strike, you know what you're going to see?
A lot more nuclear terrorist strikes. What does a terrorist organization care? They just take the resources and money from governments, they don't live in the country. If the Soviets make noises about entering afghanistan, they'll walk into Iran.
Not that the USSR would get the international go-ahead from the US to invade another country, because what's the guarantee to the US that the Soviets aren't just going to attack, find what terrorists they can, and then set up a puppet government, which is suddenly on the list of "don't mess with or we kill ourselves.' In an age of paranoia and distrust and heightening tensions, who would believe the Red Menace would just have held selfless intentions?
Yes, a single nuclear blast is a pinprick to Russia. Russia is too big and has too many people for any single blow to be catastrophic short of your planet killer. Think 9-11; it did damage. It killed short of 3,000 people and cost a lot of money.
So what? 3,000 out of approaching 300,000,000 is insignificant. The money? You could drop it in our debt and no one would notice after a little while. This is the scale of a single nuklear attack on the Soviets, and yet something this tiny would have them kill themselves, defeating the real purpose of retaliation?
And you still haven't shown me that any Soviet leadership would be prepared to go through on the threat. Remember, the possibility of accidents is always there, and with conventional nukes you can follow a tit-for-tat response for such accidents. The Soviets no longer have this option. The Soviet leadership may have been many things, but insane isn't one of them. They want the Soviet Union to
live, and the best way to do that (to an extent) was through a nuclear offensive capability, which as a result led to MAD.
The Tsar bomb you describe? I can only see someone with their backs to the wall using it. And by backs to the wall, I don't mean a glancing blow. I mean no longer having any chance for military victory or a return to status quo ante bellum. A very possible chance is that the military seizes control of the nukes in a coup of during rising tensions, making it impossible for a radical leadership to trigger the bombs without military consent. And militaries, by their very nature, are conservative and don't exactly support bombing themselves at the slightest provocation.
NATO disbanding strikes me as ludicrous as well. The Red Army is still there. Nuclear threat had already been there. The US can still threaten nuclear strike if the Soviets attack. You haven't gone beyond MAD. You've just limited the Soviets to MAD-only response.
In my eyes, nothing can turn out well for the Soviets under this situation.
1. They are trying to hold the entire world hostage, not exactly a good thing for making friends. All the US has to do is say "We only strike back at those who hit us. The Reds, they'll kill you if some nutjob hits them." Expound on it, and the Soviets are going to wonder why all those people who believed in there benevelence have been pushed to the side lines. Threatening everyone if they don't leave you alone pretty quickly gets you left alone, which isn't what the USSR really wants. It wants to spread its ideology and influence, not lock it in a box no one else can approach.
2. Can you say insurgency? If conventional assault is out of the picture even more than before, the US may decide to restart a European insurgency, and try to work out the kinks in the process. The Soviets can't really kill themselves for something so small, since they do all across the world. But eventually, the insurgency can gain strength and what do you know? The Soviets have yet another Vietnam/Afghanistan/nasty insurgency in Germany, or Poland, or, or... The cost for the allies to do this will be less than it was prior to the planet-killer, and the cost to the Soviets will easily outweigh the cost of the nuclear arsenal they tried to bypass.
3. Considering how no one attacked the USSR with tanks or nukes OTL, the tensions of the European front have two possibilities: no change or higher tensions. NATO will still be there to defend against the Red Army. But now that the Red Army is going to have those "massive" new funds to play around with, they'll probably be much stronger. Remember, doubting nuclear threats is two-sided. The Soviets may become convinced that they're Army can overrun Europe before the Americans arive (always a threat OTL), and may also believe that the US won't have the will to carry out their threat. The potential awards may outweigh the risks.
But continue on at that thread for a moment. The Reds invade, and the US reponds with tactical nukes on Western soil. No one has bombed the USSR or even its allies technically, and only replaceable troops were lost. The USSR can't respond with tit-for-tat tactical nukes on Europe. "The Line" hasn't been crossed, since neither the Soviets or its allies have been directly attacked. The Soviets aren't going to kill themselves because an invasion isn't going as hoped.
And what if they keep fighting? Then they've commited to fighting for the prize, and have locked themselves out of their threat. As improbable as it is, let's say NATO throws the Germans back after a tactical nuke. The Soviets didn't kill the world, but neither have they agreed to an armistace. That means the conventional war is still on, and so NATO does its best to role into East Germany. I doubt there would be as much civilian resistance, and for the sake of the argument lets say that NATO liberates Germany without the world dying. The situation has become a major loss for the soviets, because they have lost Germany. They can't get it back by nuclear threat (a planet-killer can't work on the threat "give me or I die"). They can't get it back by conventional, either, for NATO has already learned that tactical nukes, when done on land they control (Germany will be immediatly reunified, I'm assuming), don't trigger doomsday response.
What could have been victory has turned into disaster for the Soviets. They've lost an important satellite in Europe. Tactical nukes are no longer taboo, all but ensuring their use in the future. At best, they can request a cease fire to end the fighting and keep what they've got (if they're being thrown back uncontrollably, then a planet-killer could be believed). But this is what I was warning about: the Soviets only had a choice between destruction or humiliation once the Americans used tac nukes. And you saw how the war escelated? Conventional invasion west, tac nukes on own territory, conventional invasion East? This is how the doomsday threat can be sidestepped; by making the Russians constantly redraw "The Line" while never crossing it.
So the Soviets have been nuked (making war nukes non-taboo in the future), and the world lives.
3. And of course, the Reds could make a threat, not have it believed, and one or more of the bombs is detonated. Then the world dies, and the "defensive" threat was about as useful as maginot lines in France:inpenetrable from one direction, useless from another.