The problem with altering the status of the temple to non-temple would be that it might be seen as legally equivalent to destroying the temple.
Of course, it the demon can't do anything about it, then tough luck. But if you start remodelling the walls to make it not conform to an attached document that defined the temple, then you are in fact turning the building from a temple into a non-temple, thus destroying it, thus, requiring the demon to stop it.
As for his ability to defend the temple, the contract is probably written so as to be valid under the local laws, and therefore local case-law. And I'll bet you that you can find case law that sends someone in jail for doing something similar.
OTOH, you might just get the local rulers to deny demons/devils adulthood, thus denying them the ability to enter or be bound under contracts, except under a ward (as per the relevant case law for other legal minors), and thus rendering the entire contract (and any other contract binding a demon) null and void.
Of course, that may backfire hilariously, as every demon bound gets freed of his contract the second he enters the country. And might get rebound the second he leaves.
But ask your DM about case-laws. And whether or not outsiders all can achieve personhood. And the precise requirements for entering a contract to begin with.
