In most cases, it is legal. Fanfiction is a form of creative expression by fans. As long as it doesn't violate any copyright laws in a major way, like using the exact same footage from the trailer without permission in a way that undermines the original work's marketability. But if it's on a legitimate platform that respects intellectual property, you should be okay.
Infinity War comic is definitely worth a read. It has a rich storyline, stunning visuals, and introduces some interesting concepts that expand the Marvel universe.
Wolverine doesn't make an appearance in the Infinity War comic. The comic primarily centers around other characters and their battles against cosmic threats.