No, you’re getting Ace all wrong. What I read from the dev blog is that Ace is being added in the revisions to the existing story, and they will appear regularly starting from early game until some undefined point. I actually think Ace’s story may already be over by the time you get to the end of content currently in E18, as the devs said that players continuing into new E19 content need not concern themselves with Ace in order to understand the new story; this reads to me like Ace isn’t involved in the ending at all, or at most a minimal role that doesn’t change the plot. My sneaking suspicion is that they are killed off at some point pre-E19.