I think it's fine to leave things up to the imagination a bit. It's enough that they give enough clues to draw conclusions from. Curse will get resolved eventually, she apparently repells bad luck, shown by her family falling to ruin without her.
I guess the parents grasped at anything that could fix their problems after thinking "how can all these things go wrong, we must be cursed! Oh, didn't we have a daughter who said our house was full of ghosts? And things were better when she was around?" Being desperate to find a way out, they'd believe anything.
Guy not having ghosts around I think is meant to show that he is righteous. We've seen he doesn't really go around killing people needlessly (he scared the parents off, didn't cut their necks), and he defends the border, he is not out there pillaging his neighbors. I guess their neighbors don't personally hold grudges when they die trying to invade. And other deaths around him are due to the curse, so some kind of misfortune. Those who died wouldn't know to hold it against him. But he grew tired of seeing people die and didn't want to form connections, so he was cold and played up the nickname to keep others at bay.
What is not explained is how the marriage was arranged, if he wasn't interested in it. Who paid his parents to send their daughter to him? He is poor, so they wouldn't benefit from an alliance and he can't pay. Did the prince mean it as a punishment for being happy?
The prince is quite a piece of work too. Already had at least four dead lovers? Meaning he was sleeping around, then killing them to avoid scandals.