Reiner did die in the manga. His death was a result of a series of events and conflicts that had been building up throughout the storyline. It had a major impact on the plot and the other characters.
Love and sacrifice are common themes. Often, Reiner has to sacrifice something for the sake of love in these fanfictions. It could be his position in the Legends world or his own safety.
I really don't know. Maybe it's a complex storyline or a sequence of events that takes up 11 hours of the narrative. Without having read the novel, it's difficult to give a definite answer.