Armin could be the storyteller. Sometimes characters take on this role to provide a unique viewpoint or to drive the narrative in a particular direction. But we need to know more about the story to confirm it for sure.
Armin is still alive in the manga. The story developments don't result in his demise. His presence remains significant and continues to shape the narrative.
There could be. Check out various fanfiction communities. They often have filters or tags that can help you find the more family - appropriate stories about Eren and Historia in this situation. It may take some searching, but they do exist.