Your assumption is incorrect. Lifting the Shadow Curse is Halsin's story. In fact, the Curse will not be removed unless Thorn is defeated and does not take hold until you go to enter Act 3. As far as the encounter you mentioned you may wish to alter your strategy or party make up. Clerics have a spell that makes this combat rather easy as long as the other toons prioritse the ranged baddies.
Under the conditions you stated, Halsin will leave when you exit the Shadowlands. You should even get a short cutscene when you do.
