Answer ... In international arbitration, the law of the arbitration agreement is that agreed by the parties. In the absence of such agreement, the arbitration agreement will be deemed valid if it meets the requirements of the law applicable to the substance of the dispute or of Spanish law (Article 9.6 of the Arbitration Act). For matters of interpretation of an arbitration agreement deemed valid under more than one law that go beyond its validity, the arbitrators are free to opt for the law that they consider most appropriate bearing in mind the nature of the dispute.
Answer ... Yes. The arbitrators will uphold a party agreement regarding the substantive law of the dispute (Article 34.2 of the Arbitration Act). If the substantive law chosen by the parties is unclear, the arbitrators will apply the law that they deem appropriate (Article 34.3 of the act).