One possible plot idea could be that Issei discovers his singing talent during a school event at Kuoh Academy. His singing is so enchanting that it even has an effect on the devils and angels present. For example, it might temporarily calm the hostilities between different factions. Another idea could be that Issei uses his singing as a form of magic, perhaps it's a new power he discovers that is related to his Boosted Gear. He could sing to heal his comrades or to break through magical barriers. And yet another plot could involve Issei entering a singing competition in the supernatural world, where his performance attracts the attention of powerful beings who want to use his talent for their own purposes.
His singing ability would likely change his relationships in multiple ways. For the devils in his peerage, it could be a source of pride as he becomes known for this unique talent. They might promote his singing, which would increase his visibility in the supernatural community. For the female characters, it could be a romantic draw. His singing could be a way for him to express his emotions in a more nuanced way than just through his actions in battles. For those who are initially skeptical of Issei, like some of the more traditional devils, his singing could be a way to break down barriers and gain their respect as they see him as more than just a typical reincarnated devil.
Issei could react with anger. He might lash out at his partner and the person they cheated with. He could also become very confrontational, demanding explanations and apologies.
Natsu might be able to sing fanfiction if they are part of a community or group that does such things. There are some creative groups where people take fictional stories, adapt them, and turn them into musical performances. If natsu is in such a group or has the inclination to do so on their own, they could definitely sing fanfiction. They would need to first decide on the story they want to use, create a melody that fits the mood of the fanfiction, and then practice singing the lyrics that they have either written or adapted from the original story.
In fanfiction, absolutely. Fanfic writers can re - imagine Naruto in various ways. Singing could be one of those ways. Maybe he has a hidden talent for singing that he discovers while on a mission, or perhaps he uses his singing to connect with people from other villages. It's all part of the creative license that fanfiction gives to its writers.
Issei could develop by learning the discipline and self - control that comes with being an exorcist. This would contrast with his more impulsive devil - side nature. For example, he might have to resist the urge to use brute force and instead use the more strategic and spiritual techniques of an exorcist.
This was a joke, meaning that others could get paid for singing well, but if you sang badly, it would kill the audience. This was an exaggerated metaphor used to describe how bad singing was. It was not a literal meaning, but a humorous way of expressing it.