Every comp here has every reason to at least stay with your group, the tadpole. While I haven't had it happen yet, they can try to kill you, and leave, too. Lae'zel actually makes perfect sense, not only did you fight with her on the ship, but you rescued her from the trap, presumably.
I agree that every companion has a reason to stay with the group; I'm just arguing that this reason (the tadpole) isn't sufficient to automatically cede leadership to the PC.
An idea I had was to, on the first or second night, there would be an automatic group huddle with all your companions. All companions would discuss the group's options with others and you, and decide upon a course of action. In this conversation (different than talking to each companion individually!), you could take a more back-seat type of role and watch the companions debate their preferences, or you could exhibit leadership and try to steer the conversation/others to do what you want. In this conversation, it would become clear that the group is unable to work together, and it is you (somehow) that keeps them together. For example, it is clear from their party banter that Shadowheart and Lae'zel would not work together if they weren't both following you. Just extend this a bit to make it more clear why/when/how the companions all unanimously decided that you were the leader. No one really trusts anyone else, but they all trust you a little bit for [whatever reason] (you're smart, you're charming, you saved them, you're weak and thus they're not threatened by you...)
And oops I forgot about freeing Lae'zel from the cage! I'd still argue that this isn't sufficient for her to grant you the title of "leader" though. Partner instead of subordinate, maybe. Although freeing her after she demands it is a very subordinate-type of thing to do...
