Ignorance isn't a defence in law
That's true, but his example isn't a case of that because doing 50MPH when the Speed Limit sign says 50MPH is, by definition, legal. Posting a 50MPH sign in a 25MPH zone and then charging violators with breaking the speed limit makes you guilty of entrapment, because you're accusing them of a crime they didn't actually commit according to the posted regulation. You don't get to go back and change the sign, either, as then you're acting ex post facto, which is also not allowed.
Paarthurnax's situation is similar to this; he cannot be charged with war crimes, because according to Dovah law he was doing
exactly what he was supposed to- keeping the puny humans subjugated, by whatever means necessary. That human agents (in this case the Blades) now seek to charge him for said crimes is ridiculous on the face of it; not only are the Dovah not bound by human laws, but nothing he did was actually illegal, so there is no basis in fact for the charges.
If anything, his fellow Dovah should be trying him for treason, since that's exactly what helping the humans overthrow Alduin amounts to.