I'm hoping this is basically like the author asked "What if the Doomslayer retired?"
I may have too much "Doom" on my mind, but the parts depicting his time in the rift just seemed like a perfect parallel to Doomguy staying in Hell to kill demons for all eternity.
I mean Erik became king for his service in holding back the Majin king, right? It was probably a foregone conclusion that Luck was going to become a noble even if he wasn't one right then and there.
He was, but that's not always a guarantee of being king, since we don't know if he was the eldest prince. Not to mention that even if you are the eldest prince, there's always a battle to be the successor among the princes so you aren't 100% going to be king. Being a literal savior of the human race is probably a good way to secure the throne, though.