Shosanna is the soul of the movie in two ways - in emotional terms, where we are engaged in her fate from the very beginning of the movie, and because her story is the catalyst for all that happens subsequently, where Tarantino's themes all come together. She is at the centre and is at the heart of what the film is about.
And also the delicious irony of the end, once the layers have built up. The propaganda film
Nation’s Pride is also a central theme. It’s a movie within a movie. There is a reversal really, an about-turn, when we see the Nazi’s
Nation’s Pride enjoying and laughing at all the Allied troops getting killed in the movie. We might love to hate them for it too, maybe think they are shallow or outright henious monsters for doing so, for enjoying it so much. And then we might find ourselves shortly afterwards doing exactly the same, enjoying it and laughing when the tables turn, when they meet an eerily simmilar fate.