Time reversal!
I realised that I had been careless in my post: obviously the count must have presented evidence to the elector already, or he wouldn't have the title. The alternative would be corruption in the Carroburg court, which I had already ruled out.
Besides, there is no way the party can make a gigantic detour to Carroburg at the moment. The Graf is expecting them to get to Marienburg relatively swiftly, to appease the angry elves.
Stupid side quests!