That seems completely absurd to be honest.
Who would think that, instead of giving your agent the legal documents, giving your legal documents to a Devil and giving the agent a scroll to summon the devil at need, is sensible idea?
You just made everything more complicated, wasted the money for a Scroll of something like Lesser Planar Ally at least and made it more complicated for any officials to check up on your agents.
If anything, an officer of the crown should arrest you for trying to pull this shit and your boss for making his people try to pull this shit.
And the devil too for being complicit in it.
The point is not that the devil would have the documents, but that it is itself the repository of whatever arcane legal exception you are using to justify your movement and the reason it can happen like this is because Chaliax is not a kingdom allied with Baator, it is a kingdom in the thrall of Baator.
To give a comparison before Rome has law tablets that everyone could read they had a tribunal of elders who functioned on a 'trust me I'm honest' principle, that obviously gave them a lot of power. Same principle with Chelish law and devils. No one in the captain's position wants to question a serious devil and you just convinced him that this is the kind of mess he was in at the moment
The improbable nature of the lie you had to go with was why the DC was so high.