They're not allowed into their own city which is part of their sovereign territory? You're putting part of the High Kingdom of Ulthuan under blockade and trying to prevent its elven citizens from freely travelling to and from their own territory. This is the fantasy British Empire you're trying to deny freedom of navigation to and one of their own cities you're threatening to starve them out of.
They're the fantasy collapsing british empire which really shouldn't be wanting to send forces here while they have an unresolved war with the dark elves going on.
If they actually show up with a significant force or threaten to declare war there are options.
The point of them intervening in Marienburg is that they want to influence the Empire by controlling the centerpoint of our trade and economy, but I'm not sure how well that's been going for them, maybe they could be brought onboard with an imperial reconquest of Marienburg by offering them some manner of expanded political rights in the empire so long as they order the Exarate to stay out of it/get their people out of the way?
Not sure exactly what they'd want in exchange for a commitment to prevent elvish interferance in the war though:
maybe the Exarate gets the right to control one Marienburg imperial electoral vote until such time as they choose to trade away that right?(under the expectation that if they don't the emperor has the option to assign a second vote to the rest of the city at a voted-upon time[everything about this solution is probably too illegal to use and requires way too much cooperation from the electors]), Maybe they'd want a commitment that the emperor will act in line with their ambassador's policy preferences in some specific field for a specific length of time, such as in using Imperial colonies to benefit Ulthuan for the next generation or something,
or a requirement that the Imperial fleets(and maybe Reiksguard?) Are required to move to aid the High King's military endeavors(or maybe just defend Ulthuan) when requested, without a corresponding obligation to defend the empire on their part(I like this one, we need the Vortex intact as much as they do, but without something like this the empire'd never get involved in any attempt to defend it), maybe the emperor commits to order a certain percentage or number of College wizards to act as agents on behalf of Ulthuan in the Old World for X period of time, or maybe something closer to being symbolic like requiring that all elves be treated as if they have the legal protections and social privileges of an imperial noble of X rank.
It's all about having something that the Pheonix king can use to shout down his detractors, and claim that he wanted this all along/has more influence over the Old World than he did before he chose to interpret their defence treaty as not applying to new Marienburg and chose to support the Empire's annexation of the place.
And we carefully ignore that, if this river attack was Marienburg, most of any conquest of the city'd probably be at the hands of dwarven armies not any accompanying imperial ones.
Some of these ideas probably sound like they've got no chance of flying because they step on national sovereignty, but I'm not sure the idea existed the way it does in the wodern world before the RL Treaty of Westphalia, the ones around Electoral votes might not work, on account of laws designed to defend Electors' rights from bad emperors, but I think the ones committing the emperor to use their powers and control over pan-imperial institutions a certain way would work.