1.) I do not believe there is any way to breakup with characters. If they have a heart, it's there to stay.
2.) There are Barricades that can be found by rummaging around the Junkyard (I'm not sure how many) but it basically blocks Ragroaches from being encountered, for the most part.
3.) I'm actually just encountering Ste, for the newest version.
4.) I'm fairly certain Northcrest has yet to be an actual area, let alone have much planned, outside of lore. As such, you cannot fight Scar more than the one time.