This is all good and nice until you have multiple different nobles with contradictory demands that keep getting dorfs jailed.
That's why I've said "highest ranking" noble. Once a higher-ranked noble moves in, all laws passed by lower ranked one are up for review. If there's a conflict between a law the highest ranked noble in the fort passed and the law passed by lower ranked noble, the one passed by the higher ranked noble "wins". Just like in the US - Federal law overpowers state law if there's conflict between the two.
There’s one wrinkle there that you might not be aware of:
Commandeering.
Basically, the Federal Government cannot preempt a state law if the Federal Government does not have the power to enact the preempting law (see U.S. Constitution, Amendment 10).
Anyways, I agree that, in DF, the laws of the highest ranking noble should take precedence. However, I also believe that nobles whose domains do not include the fortress should not be able to make laws regarding the fortress. Nor should they be able to issue mandates or demands (nor should they even have room requirements). Preferably, they should emigrate from the fortress and go to wherever their domain is (even if it means taking their immediate family with them).