Historically, most polities that impose a blockade like this, blocking off one of their major trading partners, ended up hurting themselves more than anyone else. When the Confederacy self-imposed a cotton blockade, the rest of the world found other sources of cotton (most famously from Egypt). When OPEC self-imposed production limits, it raised the cost of oil so high that previously unprofitable oil reservoirs suddenly became profitable. Now the world's largest consumer of oil actually exports oil instead of buying from OPEC. Yes, the Marienburg blockade will be annoying in the short term, but even if the canal projects fail, the natural response of Empire citizens finding new trade routes and enlarging existing ones to handle the sudden increase in demand will put the Empire in a better position relative to Marienburg, and make their return more likely in the future.
There's every reason to expect the Empire to be able to weather this tempest in a teapot. Even the vote options make it clear that the Empire will survive this. It'll weaken the Empire, temporarily. Not kill it. Yes, individual Elector Counts are currently dealing with a number of problems. But the Empire is not. The Empire is not facing any immediate crises, which makes this the best possible time to go through with this. Individual Elector Counts will always be dealing with a number of crises. Having powerful Elector Counts who can be relied on to deal with local problems with no or minimal aid from the Empire is one of the few strengths of the system.
Shoot, if Marienburg really imposes a blockade, that just incentivizes all the existing river trade companies to invest in facilities to ship through the new canals when they open. Suddenly all the wealth that was flowing to a self-declared enemy of the Empire will be flowing into one of its, and our, allies.
[X] They can make up for the loss in trade
Also approval voting this because anything is less stupid than converting an economic gambit we're winning into a war we might lose.
[X] For sufficient concessions