I don't think we know how wiling Marienberg is to come to the table, or how willing the Empire historically has been to negotiate. I think there's evidence that the Empire has previously signed treaties acknowledging Marienberg's independence and then broken them by attempting to reconquer them by force.
Do we know that the Empire has even tried making reasonable offers to Marienberg, or whether revanchist forces within the Empire have seized on the opportunity to keep escalating the dispute in the hopes of manufacturing an excuse to invade, hoping that the Colleges mean they'll win this time? For all we know there are people blocking a diplomatic solution within the Empire, and that's part of why Marienberg is getting desperate. We really don't know much about the recent context or motivations of the major players.
The guarantee doesn't need to be in terms of military intervention. For example, on the dwarven (and possibly Kislev, if they were involved), it could be a promise to close their canals to trade/vessels from anyone who breaks the treaty, which is also a substantial deterrence. On Ulthuan's part they could ban Marienberg's vessels from Lothern.
I'm also not that convinced that the Norscans need to be paid by Marienberg to raid the coast of the Sea of Claws. I think they'd be carrying off (relative) innocents anyway, and the Empire's coasts are some of the most convenient. At most, I think Marienberg's payments would persuade them to raid the Empire rather than the Wasteland's coasts.
There's also the issue of the major opportunity cost of a war with Marienberg. Given the great amount of territory that the Empire has lost to the beastmen and greenskins, it would seem like it makes more sense to focus efforts against enemies who can't be negotiated with. Marienberg is likely to be a very hard target, so we're better off steering the Empire at better targets like fallen nexuses. Conflict with Marienberg is in competition for resources with our wider objectives, I feel.