Tarquin only came into Elan's life, what, a week ago? Few people would have thought "Reconciling with his father, the mysterious evil overlord who is probably Lord Tyrinar, is an essential component for This Story to have a happy ending for Elan" before Tarquin appeared on-panel (and not in a flashback). This Story--the story of the Order battling Xykon over the Gates--will have a happy ending, for Elan, at least. That doesn't mean there won't still be things about the world that make Elan sad when he thinks about them, such as what utter rat-bastards his father and brother turned out to be.
Tarquin is Xykonishly evil. He is not going to suddenly have an epiphany and go, "Oh wow, that was really wrong of me to torture and kill all those people!" And Elan's sense of morality is not so shallow that he wouldn't recognize, e.g., sticking a Helm of Opposite Alignment on his father's head, as a poor substitute for him actually reforming. So there is no way Tarquin (and Nale) can possibly contribute to a happy ending for Elan. Whether he looks back on them and thinks, "It's so sad that we had to kill my father and brother," "It's so sad that my still-in-power father just raped his twentieth wife-by-torture," "It's so sad that my father's going to be in prison for the rest of his life," or, "It's so sad that I had to brainwash my father to make him Good," he is not going to think "It's happy that my father [anything]." It will always bother him that his father is/was a monster as it bothers him now when he remembers his mother crying about having lost a "nail." Such is the level of nuance of a story with a happy-but-not-Strawberry-Shortcake-level ending.