We would need to act according to basic compenetenceThe problem is that the Coin doesn't make you supernaturally convincing, it just makes people believe you believe what you are saying.
Like, in real life, I believe that the dude who stands on street corners yelling about a conspiracy to destroy the country believes what he is saying. I do not think he is trying to trick me. But I also think he's, to put it charitably, off-base. If a complete unknown starts spreading rumors, then even if people believe she isn't lying, they still have no reason to trust her information; they'll never believe you're lying, but they might believe you're wrong, or just crazy. So I don't think we could stir up discontent in Marienburg using the Coin; at least, not without the benefit of pre-existing contacts or other avenues for disseminating information.
Did I say that we should shout on the street like a crazed loon? Please don't assume that we will do it in the stupidest way possible
We could put actual effort into it. We use a disguise to just mention it at a bar that we say heard an elf say that the King would never support Marienburg. We then as someone else mention something similar, repeat, pretend to be multiple people. Later pretend to be a wealthy merchent and mention they heard something etc.
If someone hears this more than once how would they react? Do you think the empire lacks agents in Marienburg that could help.
We spread as many different rumours as possible. The goal is to make people doubt that messing with the empire at this time is a good idea...