The strange narration in the second half of Trust Exercise

To recapitulate: The first half of Susan Choi's novel Trust Exercise (2019) tells of some teenagers in s high school for the arts and their various rivalries and amorous missteps and mistakes, with a focus on two, David and Sarah. This section ends abruptly after a very bad night for Sarah. The second section, in itself called Trust Exercise, takes place in the late 90s, about 15 years later, as Sarah, a successful author, reads at a bookstore from her new novel - presumably, what we've read in the first section; lurking at the edge of the audience is her high-school sometime friend, Karen, who seems to hold an enormous grudge against Sarah. S is upset to see K, but she sort of hides that and the two go out for dinner and catch-up. K does most of the talking, catching S up on some of the people they knew back in h.s.; K is still living in her hometown and is involved in the theater company that David runs and has taken up acting again and will play a lead in a new production (which sounds dreadful, but who knows), starring alongside an English actor, Martin, who played a tumultuous role in the h.s. days of section one. OK. This can work, so long as Choi can build some drama of her own: Is K. a stalker? Is there something we don't know about her teenage friendship w/ S., something that could make her hold such a fierce grudge over all the intervening years? Did S get something entirely wrong in her novel, something about which K will inform her? We don't (3/4 through the novel) know yet: K is an enigma and S is nothing but a listening post in part 2. Mostly, Choi is playing with the edges of narrative: Don't trust the author of a novel to tell you the truth; novelists are risk-takers who live on the edge and are bound to offend someone or everyone. For reasons completely unfathomable (to me, anyway) Choi switches, sometimes in mid-paragraph, from first- to third-person narration throughout this section: Sometimes she describes Karen; sometimes Karen is telling her own story. This decision makes no sense to me. By this point in the novel, the narrative should settle on a POV and move forward; if there are mysteries to unveil and truths to be uncovered and revealed, time is running out.