As the decade-long legal battle over Apple’s past Irish corporate structure enters its final phase, the €13 billion in disputed corporation tax sitting in an escrow account has become a mere by-product of the case. What is really at stake in the appeal heard on Tuesday by the Court of Justice of the EU is a clarification of the rules the European Commission can use to police the use of tax incentives. As its lawyer Paul-John Loewenthal put it at the very start of his plea, the case is of utmost importance for the future of fiscal state aid and…