@Terithian I'm just guessing here, but I believe that even if it came out that the order was a forgery, the time it'd take to absolutely verify it may be too long for the guy to do nothing, which would then risk him being labelled a traitor for not acting upon it. While it may seem stupid, there is a measure of reason ability to it, in times of emergency the King/Queen would need to be able to give orders holding their full authority even when they cannot go there in person, if you establish precedence that it's 100% okay to ignore those orders when you
think they're forged then you basically open your country open to attack because now it comes down to a lawyer's game of how reasonable can you make your suspicion. While forgeries/thefts can erode the authority of the seal, you already have to prevent forgeries/theft of the seal separately, allowing traitors to hide behind the
belief that an order is illegitimate in order to sabotage important emergency orders erodes the authority of the seal through a different means entirely. Basically, the seal becomes worthless if he wouldn't face punishment if word got out that he received the fake order but did nothing, thus the options presented were to go along with with as many token efforts as they could, or kill the Princess' group and try to cover up the existence of the fake order in the first place.
I will admit though, that if this is in fact the reason, that it should have been explained why they didn't have time to prove that it was fake, since the idea that it'd be impossible would be pretty unreasonable barring this specific set of circumstances creating it (a newly appointed lord claiming that a Princess who is a potential heir to the throne in this time when the various heirs are competing with one another getting into a legal argument with one another could potentially be an extreme case where either he wouldn't be able to convince anyone, or even if he did he'd put himself in extreme danger as the judges may decide to try to win favor with who might become the next queen).