SPOILERS: A subplot involves the court trial of a serial rapist, a character that the reader knows is guilty because of his confession in book one. The defense team tear into the survivors who testify and ultimately he walks free. When one survivor hears the news she breaks down, destroyed, and it's quite distressing to read.
An eight year old girl is taken from her garden, but she's found safe. The end of chapter 36 also mentions a decades-old case of serial kidnapping and murders.
There are one or two passing references to Pip being in the hospital in between this book and the last book, but no scene set there. There is also a scene in the final chapters of the book where an ambulance crew is called out and performs CPR.
The mental stability of a character is questioned, but we the reader are quickly reassured that this character is not a danger to themselves. Later there's a passing comment about a parent ending their own life after a horrible tragedy. It happens long before the events of the book and is only mentioned the once.
No, but a man calls an overweight person a "fat loser". This person is also self-conscious of their body and is given a fitbit by an unsympathetic parent.
I wouldn't 100% call it sad. There are some positives that the book ends on, but mostly it feels raw. It's clear that healing needs to - and likely will - take place after the events of the plot.