Mediation Settlement Agreement (Oregon)

Takes place in Oregon - Agreement reached for partial amount of note balance due; owing party failed to sign agreement until almost 18 months after other party died and still it is undated. Agreement has become part of estate, but wondering if it is even valid since only the note holder had signed and dated the agreement. Can the heirs sue owing party for full amount of note balance since they believe the agreement is now a moot point?

They can yes unless the language or some agreement says that the mediation was full and final settlement of the issue.