I think it was useful in that it showed a certain "someone needs to stop these people" side to Amanda's initial motivation. On a practical level, knowing that the Graysons are still killing people who could expose them doesn't leave Amanda with many choices. If the Graysons ever found out she was rich again, she would register on their radar and would be in danger.
(3) David's alcoholic friend was going to help Amanda, got murdered, and then Amanda went all Dark Willow
To be honest, #3 was both interesting and unnecessary. I had no problem believing that Amanda would want to avenge her father's situation, even without the motivation of his only friend being murdered. I guess the show wanted us to see how she came around to believing her father was innocent and what made her decide to take everyone down, but this episode seemed like it was treading water.
Jack's hair was especially silly in the very first scene, with that hat/cap/I-was-laughing-too-hard-to-see-what-it-was. Amanda's dreamy eyes and the music were comedy gold in that context. Unintentional, I presume.