Scary Story Time

How implausible is it that Russia possesses the capability to successfully imitate an Iranian submarine?
Diesel/electric boat presumably, something that puts out a mix of known Iranian boats drive and powerplant noises. Have it show up on US sonar technology initially somewhere in the south Atlantic. Finally goes silent west of the Azores maybe. Briefly heard for the last time due south of Cuba. Two to three days later a complex nuclear device detonates 100 miles due south of New Orleans, Louisiana. The resulting tsunami would stretch from Key West, Florida in an uninterrupted wall of water all the way to the north-eastern point of the Yucatan peninsula in Mexico. Cuba wouldn't actually cease to exist.

I sincerely doubt Iran has the capability to even approach pulling off a scenario like this, but I'm reasonably certain Russia does. 

I have heard others now describing what's happening in and around Ukraine as forcing Vladimir Putin into deploying tactical nukes in Ukraine, if only to have some expectation of surviving. At all, not just politically. According to that Fount of All Knowledge, Wikipedia, tactical nukes can range in explosive power between tens of kilotons to hundreds of kilotons and delivered by hand or, as noted above, torpedo. If two 500 KT tactical nuclear devices were situated approximately 200 feet from each other on a true east/west alignment and detonated simultaneously, the physics of the resulting mushroom cloud would make clear the number and thus the size of the devices involved to anyone 50 miles away or so. The point is, Ukraine might not be the most advantageous spot on the surface of the Earth for Vlad to pop off a nuke.

Plausibly pointing the finger of guilt at somebody else is a critical touch for long term success, but Vlad's almost 70, so maybe a Highly Important secondary consideration then.

To the extent any of these maunderings has any such thing, the point of all this is that, as the level of violence deemed acceptable ramps up, so too does the range at which inflicting that violence reach further. If Vladimir Putin decides that the end of Russia as an independent country is a possible result of the combat in Ukraine, then the potential target list expands beyond that country's borders. This is the point of the decision tree where things start to get messy.

A plausible story is essential to success. Obviously, the more plausible the better, but how much is enough? In how much time? Added to this, tactical nukes usually don't require National Command Authority for tactical release as prescribed by doctrine. Only Putin can decide if/when to detonate this weapon though, which circumstance already attracts too much attention as is. Presumably Russian bureaucratic lethargy is no worse then our own, so the mascarovka should mostly be in place. 

In order for a plausibly deniable story to exist the supporting elements have to be put in place which takes time even in bad fiction. If the Russian Navy needs 10 days to do their part, that means the Russian Army has to wander around Ukraine not winning anything for an extra week or so, at least. If that all starts to look a lot like losing, the device is already in place.