Presumably Zanna is moving out because she is the only one left of the three girls who lived there. Joan is dead and we can assume Sunnie is in jail. Paying all the rent would be hard for Zanna since she doesn't have a job. If she's staying in the area, she would need to find a cheaper place to live. It's possible she applied and was accepted to another law school, the movie doesn't say how much time has passed since Joan's death and Sunnie's arrest.
It's entirely possible she would be given immunity. I think she probably saw an attorney before coming to visit Sunnie that last time, or maybe confided in her former professor. Remember, he saw her in the college library and asked what she was looking up. We see in the movie that she threw Sunnie under the bus, so to speak, to save herself. She knew she was doing a terrible thing, but she cared only for herself.
I think any guilt she feels was pushed down and buried. I don't know if she could have gotten into law school with that on her record, even with immunity. Just at the end, when she watches the video about friendships, I see her as struggling with what she did but knowing she can't ever take it back.