"A citation may support a nearby sentence rather than the claim the user cares about" is the failure worth building around, because it survives every check that only asks whether citations exist.
The part I would make explicit in the contract is the unit of attachment. A citation on a whole paragraph is nearly unfalsifiable, while a citation on a single claim can actually be verified with an entailment check.
The stale-source case deserves its own clause too. A document being authoritative and a retrieved passage being current are independent properties, and only the second one degrades on its own while nobody is looking at it.