Yes, there might be. The Naruto and Attack on Titan fandoms are both huge, and fans often like to cross over their favorite characters in fanfiction. So it's quite possible to find Eren - from Attack on Titan - in Naruto fanfiction. However, it may not be as common as fanfictions within each respective universe.
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.
It might be. Since both Naruto and Attack on Titan have huge fan bases, a fanfiction like this can attract a lot of attention. The idea of Naruto, with his unique abilities, stepping into the world of Attack on Titan to save Eren's mom is full of potential for creative and exciting storylines. For example, Naruto could use his shadow clone jutsu to deal with the Titans while trying to reach Eren's mom in time.
Fanfiction about Eren and Levi kissing can be quite interesting. It might explore a different side of their relationship. Some fanfictions in this regard could be about a moment of emotional intensity between them, where the kiss represents a deep connection that goes beyond their usual interactions in the original story.