You can register the code and just not do the story.

The exp boost earrings aren't that important. Experience is not hard to get in this game and there are many other experience boosts, such as the armory bonus, the bozjan earring, the brand-new ring, the heat of battle free company buff and food.

If you think you will take two years to get to EW, by then a new expansion will be on the way and EW will come free with it, so unless you are getting it to access an older expansion like Stormblood, there isn't much point if you are planning to be that slow with the MSQ.