Yeah, the issue with Marienberg, as I understand it, is that the Empire wants to renege on the treaties it's signed essentially out of pure revanchism.
The best solution here is for the Empire to give up on reconquering Marienberg and for Marienberg to receive guarantees of its independence strong enough that it doesn't feel the need to play games to strengthen its own independent deterrence.
The problem with this is that the Empire doesn't want to give up its territorial ambitions and also, as the Nordland-Laurelorn issue demonstrates, has a poor record of treaty compliance with neighbours that aren't able to force them to at all times.
In many ways the problem with Ulthuan's guarantee is that it's too ambiguous and not strong enough. If they could credibly absolutely guarantee Marienberg's independence then Marienberg can drop their embargo and get on with making money secure in the knowledge that the Empire can't attack them. The issue is that they're currently too insecure and the Empire too untrustworthy.
In terms of protecting the Waystone network then having Marienberg join the accords and the Empire swearing of reconquest is the superior option. It's probably the superior option for the prosperity of the people of both nations and the wider world, as well as for the defence of the world against Chaos.