The main difference is the 'lemon' aspect. In regular 'attack on titan' fanfic, it usually focuses on the adventure, the fight against the titans, and the general plot development. But 'attack on titan lemon fanfic' often includes some romantic or more intimate relationships between the characters, which might not be the main focus in the normal ones.
There's a chance it could be different. The anime might want to surprise the audience or adapt the ending to better suit the medium of animation. But we'll have to wait and see.
No. The 'Attack on Titan' series has a set of official storylines, characters, and themes which are mainly centered around the fight against Titans, the mystery of the world, and the complex relationships among the characters. 'Attack on titan fart fanfic' is likely a fan - created and very non - traditional take that has no official standing in the series.
No, it's not possible in the literal sense. Attack on Titan is a fictional story, not a sentient being with the ability to read or watch fan - created content. Fanfics exist in our world for fans to explore different ideas about the Attack on Titan universe.
Well, it's a really strange concept as Attack on Titan is a fictional entity and can't actually 'watch' a fanfic. But if we were to be creative, maybe it would be like the characters in the series are observing an alternate version of their own story. They might be shocked, amused or even inspired by the different plotlines and characterizations in the fanfic.