Well, Saitama All Fiction could potentially refer to a fan - created idea that emphasizes Saitama's ability to be above all the normal rules of the fictional One Punch Man universe. For example, his strength is so overwhelming that it's like he exists in a realm of his own, where the typical limitations of the story don't really apply to him. He can defeat any foe with just one punch, and this 'All Fiction' concept might be an attempt to explain how and why he's so powerful in such a ridiculous way.
In the realm of 'dark saitama fanfiction', it could be a way for fans to deconstruct the typical heroic image of Saitama. For example, it might depict Saitama being haunted by his past, or having to make difficult choices that go against his original carefree nature. This type of fanfiction might also explore themes such as isolation, as Saitama's overwhelming power could potentially lead to him being estranged from others in new and darker ways.
Not necessarily. The concept of 'the strongest' is relative and depends on various factors. Saitama is definitely a force to be reckoned with, but there could be characters in other stories with equal or greater strength and abilities.
Genos could be an unexpected opponent. He might be forced to fight Saitama due to some mind - control or a misunderstanding in the rules of the tournament. It would be really interesting to see how Saitama would handle fighting his own disciple, and how Genos would fare against his master's incredible strength.