Another factor to consider regarding the Trade Compensation course is that no matter what we do, Marienburg will be running intrigue actions against us unless we capitulate, so we'll need to keep that in mind as well.
Direct sabotage actions against the canals are the most obvious choices, but I expect most of the Marienburgers to realize that the Dwarven workers are not the weakest link in the arrangement. I imagine those who fail to realize this will be allowed to accomplish whatever they can and then cheerfully offered up as scapegoats when it all comes crashing down around them.
Similarly, intrigue actions against Dwarven interests in the project are unlikely to bear fruit, and while intrigue actions designed to sow discontent amongst those shouldering the economic cost could probably have some effect, we just briefed the Chamberlain about how unlikely that would be to lead to the decisions of the higher-ups being overturned. The weak-points in the arrangement would be, as ever, on the Imperial side.
One avenue open to Marienburg would be to sow discontent. Agitating peasants against a deal designed to enrich nobles and burgers at the expense of the little people is, uh, not difficult, and arranging for """unfortunate accidents""" to befall the replacement overland trade routes would only exacerbate the issue. Peasant discontent wouldn't be enough to stop things on its own, of course, but it would increase the pressure on the various ECs to say "no".
Alternately, Marienburg could genuinely increase the local demand for, say, military equipment beyond available supply by increasing the local "banditry" levels, or, depending on how spicy they wanted to get, bribing and supplying as necessary to increase greenskin/beastmen/cultist population. I don't think they'd be stupid enough to try the latter, but it is money that's on the line. (And, if we're unlucky, we could see perfectly natural upticks in those threats which would serve the same purpose.)
Another option would be to bribe the shit out of the internal trade partners to kick up a fuss to the nobles, ensure their own shipments go "missing", or otherwise make a nuisance of themselves. We've already seen the less-than-stellar moral fiber of Talabecland when it comes to trade, and they are, presumably, not alone.
And then there's direct bribes campaign contributions favorable future trade agreements that could be offered to nobles up to and including ECs to stonewall the issue. Ultimately, while the Karaz Ankor can provide monetary compensation for lost trade, they can't fully eliminate the impacts of the lost trade routes, so if the political will to take those costs on the chin can't be mustered Marienburg wins.
Of course, the Empire does have the Grey College to fight these fires, but the fact that Talabecland is not currently perfectly behaved and bereft of half of its nobles and merchants shows that the Grey College can't be everywhere and can't deal with these problems instantly. While the Empire is not helpless to Marienburg's intrigue apparatus, Marienburg could certainly do damage with it, especially with the power of their infinity^2 money.
All of this seems to point in the favor of the blockade breaker option, because it's hard to shiv an ironclad, but there is one potential avenue I'm concerned about there, and that is to bait the Grudgy Dwarves. If some Marienburgers could fake to the Dwarves that the Elves were interfering with the counter-embargo, and a particularly ornery Dwarf captain could be persuaded to do something impolitic to the Elven quarter, that would be, uh, poor. Alternately, if the Marienburgers just chose some sacrificial ponds to throw at the Dwarves until one of them attacked something that wasn't a Naval asset and called upon Ulthuan's treaty, that would lead to issues. Now, Dwarves are a resilient lot, and I don't think that such a a ploy would be likely to work. Why, I think it would be an outright longshot.
But. Well. We heard about Ulthar's brothers.
It only take one fucker in the wrong place at the wrong time to bring it all down.
In general, I still think the trade deal is far more vulnerable to Intrigue shenanigans, but the most dire, multiple critfail-tier potential consequences of the blockade option feel a lot higher to me.
