Time to analyse the options.
[ ] The High King could make it so
This is likely to cause discontent within the Karaz Ankor.
This is very bad. Belegar and Thorgimm are already deeply divided, to the point Belegar worried about causing a civil war. Barak Varr being there too? Lordy. That might actually kick things off.
[ ] For sufficient concessions
This would weaken the Empire.
We are loyal to the empire and don't want them to be weakened.
[ ] They can make up for the loss in trade
This would weaken the Karaz Ankor and the Empire until the canals are complete.
This, however, is only a temporary weakness. And it ends with both stronger than before. So long as they do not have to face an existential crisis before the canals complete, they'll be fine and even better. IIRC, there is no existential crisis planned for the likely duration of the canal building. This also has the benefit of not wasting lives.
As such this is my favored option.
[ ] No, but they could help break the blockade
Barak Varr is the main force behind the project, and would not hesitate to deploy their navy in support of the Empire's to reopen trade routes.
Wait. Stop. What the fuck? Blockade? A blockade is very different from an embargo. A blockade is an act of war. That would mean Marienburg would actually be going to war with the Empire over this.
That changes things, if it's an actual blockade, then the war is there already and this is the option that should be picked. I hope it's just a turn of phrase.
[ ] No, but they would fight beside you
This could easily be spun as Elven sabotage of a Dwarven project. Dwarven pride would have the Throngs march alongside the Imperial Army, and Marienburg could be reclaimed.
Do you want War of Vengence 2: Manling Boogaloo? This is how you get War of Vengence 2: Manling Boogaloo. Do not pick this.
[ ] No, they cannot be dissuaded
It's hard to say what this would result in.
Ominous sounding! Lets not!
[ ] I don't know
Wash your hands of the matter.
The cowards way out. Especially since we know that at least some of the others are true.