What you say here suggests a rather big lack of understanding of history.
> For a political statement, a nation could make it very clearly demonstrated you have, say, a dozen. ... It'll cost two working models and you need perhaps a dozen packed up and well hidden to be effective. I would imagine the reaction would never be declassified, and I'll be surprised if it hasn't already happened.
This is the logic of nuclear blackmail, and in general of brinkmanship. It has been used before, if the relevant Wikipedia page is to be believed.
It is a dangerous and unpredictable threat. The Cold War strategy asserted that only mutually assured destruction would be sufficient to bring about peace. A dozen nuclear bombs, under that logic, is not enough. If it were, the results of the Cuban Missile Crisis would have been a lot different. If it were then the US would not have built its huge arsenal of nuclear warheads - 12 per nation would have been enough.
You say "always think outside the box", but nothing you've mentioned is outside of the box. In fact, it's boringly in the box. First off, there's nothing special about containerized shipping. A 1 MT bomb weighs less than a ton, and you could bring that in via tramp freighter or even private sailboat.
It's almost needless to say, but we also had a lot of cargo ships in 1900. The Halifax Explosion of 1917 was equivalent to about 2.9 kilotons, and that was with conventional explosive. Blow up that ship at a wharf in NYC and it will take down skyscrapers.
For that matter, look to drug smugglers. A single speedboat in June was captured, with 3 tons of cocaine on it. Narco-submarines are believe to carry in several hundred tons of cocaine per year. A 49-foot sailboat was recently intercepted 400 nautical miles off the US coast, on the way from Venezuela to Canada with 621 kg of drugs on board. Any of these are enough to deliver a megaton bomb.
"More than a dozen years ago, how would Saudi Arabia knock down skyscrapers in NYC? That's insanity, they'd have to launch cruise missiles or build a battleship and successfully park it off the coast of NYC, or invade overland with bulldozers or something."
This is not true. You need only to look to the movies of more than a dozen years ago to learn about some alternatives.
"The Peacemaker" has a Yugoslav named Dušan Gavrić who comes close to exploding a nuclear device at the UN.
"The Sum of All Fears" has a group of Palestinians who set off a nuclear bomb at the Superbowl in Denver.
I pointed out movies to show that the alternatives were part of popular culture, but I could stick with real-life.
The W54 warhead weighed about 23 kg and had a yield up to 1 kiloton. It was used to make a backpack nuke - the Special Atomic Demolition Munition. This could take down a few blocks of NYC.
And of course the first attempt to take down the World Trade Center was in 1993 using car bombs. That failed, but in principle that could be used to take down a skyscraper.
So in real-life and in the movies there were many other options beyond cruise missiles, battleships, or bulldozers.