Both Finnick and Prim die for reasons; you can’t have a war narrative without loss. Katniss’s sanity is also among the victims. It’s not a cheery book but it’s a fantastic and groundbreaking book as it stands. There’s a lot that will be fantastic on film without any changes. The points you suggest all involve massive changes to the story. Thankfully, Lawrence is making Mockingjay as Mockingjay, not as some love triangle Capitol trash.