Isaac refers to Abby and her friends as the "Salt Lake Crew", meaning they were the ones associated with Fireflies in Utah.
Manny
It doesn't say specifically, but this is likely Manny standing over Jerry's dead both with over when Abby finds her father dead on the floor.
Mel
In Mel's note to Abby, Mel says that her father has been an amazing mentor.
Abby tells Mel that she was her father's best student.
Abby: "Well, for what it's worth, my dad always said you were his best student"
It's also believed that the person's voice on this recording is Mel.
Owen didn't want to kill Joel. He only went along with it because that's what Abby wanted. Right before Joel's death, Owen tried to convince Abby to turn back. You could even see it in his face during the Christmas flashback scene that he wasn't fully on board with going to Jackson.
As I explained earlier, they're the Salt Lake Crew, meaning they're all ex-Fireflies. Being directly linked to Abby, her father, and the Fireflies in Utah, they should be enough motivation for them to go after Joel. We already saw what happened at the end of TLOU and there's no reason to go into further development since people are aware of what Joel did.
In the first game, David went after Ellie and Joel because they killed some of his people are the college campus. There was almost no build-up at all.