Why is she willing to follow me? Why does she put up with not heading straight for the Creche?
She appreciates your leadership qualities. She says something to that effect if you take her place in the Zaith'isk (Which actually gets approval from her).
She's clearly not accustomed to being a leader, likely due to the fact that she's young and low rank. She's used to receiving orders from superiors, so she's willing to follow someone who shows similar qualities to those superiors.
Even if it delays her main objective, you are showing strength in what you do (Either massacring the goblins or the tieflings, taking on Ethel, going through the Underdark etc.) so she defers to your judgment. Especially as her urgency is quelled by the Dream Visitor indicating their protection from immediate ceremorphosis.
Why doesn't she kill SH immediately when the artifact is revealed?
Because she knows that the artifact is somehow keeping the party from becoming Mind Flayers. She doesn't want to simply hand it over to the Gith - Though it creates a moral dilemma for her putting her duty to her race vs her self preservation instincts (Which are strong enough that she puts up with a bunch of non-Gith with tadpoles in their heads in order to receive aid to get to a creche for a cure)
Also, she respects Shadowheart's abilities. She puts value in those that perform to a high standard, that's why she doesn't care for the Gith that she killed while training they were weak and so insignificant. Shadowheart shows adequacy, heck she almost kills her during their camp scene.
I've tried fitting her in but I've never managed to keep her past a certain camp cut scene where I choose to stand back.
You should try to stick it out. She literally explains everything over the course of the game.
