It's hard to say exactly. Maybe there was a hidden plot twist or a secret they discovered that made them run. Maybe they were trying to avoid being caught by some villains. Without more details from the visual novel, it's tough to be certain.
One challenge could be acceptance. Since Ichigo is from the human world and a Shinigami, and Nemu is an Arrancar, their union might not be easily accepted by either the Soul Society or the Arrancar community. Another challenge could be communication. They come from different backgrounds and cultures, so understanding each other's values and ways of thinking might be difficult at times.
The plot could involve Nemu saving Ichigo's life during a fierce battle. This act makes Ichigo see Nemu in a new light. They begin to spend more time together, sharing their past traumas and dreams. As they fall deeper in love, they decide to get married. However, they have to keep it a secret at first due to the political situation between Hueco Mundo and Soul Society. Eventually, their love story becomes an inspiration for others in both worlds.
In my opinion, both have their merits. The anime brings the characters to life with animation and sound, but the manga allows for more imagination and detail in the panels. It really comes down to what you value more in your entertainment.
Bleach is not a novel. It's more famous for its manga and anime versions. The manga format allows for a more visually dynamic and action-packed presentation of the story compared to a traditional novel.
It's hard to say if it's common. There could be many people named 'bleach' out there, but we don't really know how many of them are fans of 'bleach' the series and also engage with fanfiction. It depends on how big the 'bleach' fan community is and how many people with that name happen to be part of it.
It could imply that a person named 'bleach' (which might be a username or some sort of self - given name) is reading or viewing 'bleach' fanfiction. It might be a strange or unique way of expressing that someone who is a fan of 'bleach' is engaging with the fan - created fictional works related to it.
Rukia would probably be very critical at first. Since she is quite serious, she might nitpick the inaccuracies in the fanfiction. But she might also secretly enjoy the ones that show her in a positive light, like those highlighting her friendship with Ichigo. In the Bleach universe, characters are used to their own stories, and fanfiction would be a whole new and unexpected take on their lives. The zanpakuto spirits might also have interesting reactions if they could read, as they have their own personalities and might be offended or flattered by how they are depicted in the fanfictions.