Can Goku use Ultra Instinct at will in the manga?

Yes, he does. Goku achieves Ultra Instinct in the manga.

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.

Yes, he did. Goku achieves mastery of Ultra Instinct in the manga after intense training and battles.

Yes, Goku has mastered Ultra Instinct in the manga.

Yes, he does. Goku achieves Ultra Instinct in the manga.

Not as far as I know. Ultra Instinct Goku is pretty much a force to be reckoned with and hasn't been beaten in the manga.

Not that I know of. Ultra Instinct Goku is a pretty powerful form and it's rare for anyone to outright defeat him in the manga.

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.

A fanfiction could focus on Goku's relationships while in Ultra Instinct. For example, how does he interact with his friends and family? Does Vegeta view him differently? In this story, Goku might struggle to control the Ultra Instinct around his loved ones because it makes him seem so detached. This could lead to some interesting emotional moments between the characters.