Starting a new character for the add-on, the game would have to assume a particular status for Antumbra and the Jackal, if they are actually in the add-on. It could just default to neither showing up unless you were continuing with a character where they lived, or they could show up by default, but not be really specific about where you had meet before or how you helped them out. The game could also just choose randomly whether they show up again or not.
It seems a waste not to do anything more with Antumbra. Her grandmother being one of the more memorable characters in Divine Divinity (though only showing up a couple times), bringing her and Buad into D2:ED could easily extend to dealing with the assassin's guild in Aleroth, or something. Antumbra herself may not be there, and her associates could assume she just didn't get out of Broken Valley before Damian's attack, so there wouldn't be a conflict regardless of whether you helped or killed her.
The FoV FAQ mentions finding out what happened to various characters, but I don't know if any of that is dependent on how you completed previous quests.
