Hard to say. You'd have to check the particular comic titles or storylines to know for sure if Darly is in them. It varies a lot depending on the creators and the plot.
Daryl Dixon isn't gay in the comics. The storylines mainly revolve around his adventures and struggles in a post-apocalyptic world, not his sexual orientation.
It depends on their popularity and the creativity of fans. If people are really into their characters and potential relationship, fanfiction could definitely exist.
I don't think Dixon Hill is a real novel. It could be a name given to a fictional work in a movie, TV show, or game, but not something you'd find on the shelves of a bookstore as a genuine novel.