See I feel Luke's moment shows much more regret than the joke he played. The Finn/Phasma scene IMO also shows much more characterization for Finn. He's clearly trying to tell the other troopers that the First orders claim about being worth anything is a bunch of bunk and we see him breaking through to them despite Phasma's scoffing. So her killing them can be used as an amplifying point for him to hate the First Order. Thus tying into Rose's final line much better. Granted it doesn't fix all the problems with his arc.