I am playing as an LS male, playing the PC version. I have completed the Leviathan, and am on Korriban as my last planet. I have talked to Carth and Mission, and their quests are now in the "completed quests" category. Yet, Lena and whoever spawns to start Carth's search for his son won't spawn. And no, I have NOT done Juhani's sidequest with Xor yet.

Can I get some help here? I want to do these quests.

You probably needed to do these before the Leviathan, but I could be wrong. Are you sure you are far enough in the conversations? You need to have reached a certain point in the conversations.

Yes, I'm far enough that when I ask Mission about it, she just says "Don't worry, I won't let my search for Griff get in the way of our mission". And I can't talk to Carth about it at all. His quest is put in the "completed quests" category.

You're screwed man. The only thing you can do is do everything over again, but I doubt you'd want to.

There's no way to force them, like with the savegame editor?:(