I think we need to look at Jacen's sacrifice as what it truly is: a test of loyalty. Anakin Skywalker/Darth Vader's test of loyalty was in Chancellor Palpatine's office, when Mace Windu was about to finish Darth Sidious. It ultimately doesn't matter exactly what Anakin was sacrificing with that act, because he proved his loyalty to Darth Sidious and the Sith. But what Anakin actually sacrificed was his dream of being Jedi, a dream he'd held since he was a child, of being the best Jedi ever. Saving Palpatine/Sidious ended that. Looking towards the future, Darth Talon's killing of her Sith Master was a test of loyalty to Darth Krayt, and a sacrifice. Both Krayt and Palpatine were looking for loyalty to them, personally, but they are also both aware that the lessons of history state they will be betrayed. That's part of the paradox of the Sith. Lumiya is also well aware Jacen has and will betray her, but he still must commit an act of loyalty to the Sith. Mara is going after Lumiya. Lumiya and Mara know each other, and no doubt Lumiya is expecting such an act from Mara. Further, I'll speculate that Lumiya is working to make sure that Mara finds her... and finds her where Jacen will come upon their battle. Jacen will have a choice... does he save his aunt and kill the one person that he feels holds the answers he needs to bring peace to the galaxy for once and all, or does he save Lumiya, and bring untold pain to his beloved uncle and quite probably the conflict he had endeavored to avoid so many months ago by killing Nelani Dinn? The business with Jacen naming himself can also be explained by Lumiya's continuous lie that she isn't a real Sith. Not being so in what she's told Jacen, she will tell him to name himself. We get Jacen exactly on his grandfather's path. Jacen thought his grandfather fell for a selfish reason, the love of a woman, but Jacen is falling for a reason made impossible by his very fall, a peaceful galaxy for those he loves. It's really the same thing. Mara becomes Jacen's Mace Windu, not a true sacrifice in the fact of the person, but a true sacrifice in the fact of what that person represents.