Yes, it is possible. In fanfiction, writers have the creative freedom to reimagine the origin of characters. Goku being born with Ultra Instinct can add a whole new layer to his character development. It could mean that his power growth would be different from the original story, perhaps faster or with different challenges along the way.
You could write a fanfiction where Goku teaches others about Ultra Instinct. Maybe he tries to pass on this power to Gohan or Trunks. This would involve Goku having to understand the power more deeply himself in order to teach it, and it would also be a great way to develop the other characters.
Goku definitely masters Ultra Instinct in the manga. This transformation gives him a huge boost in power and allows him to take on even stronger opponents.
Well, it depends on the author's imagination. Some fanfics might show ultra instinct as an overwhelming power, while others might have a more nuanced take.
Well, with Ultra Instinct, Goku would be able to react extremely quickly to the DC heroes' powers. For instance, when facing Flash, who is super fast, Goku in Ultra Instinct might be able to match his speed or even anticipate his moves better. This could lead to a mutual respect between them as they test each other's limits.