She's not a writer or director, that would be Christina Hodson (who they had already hired to write Batgirl and then, after they'd seen what she'd written for BoP, hired her to write The Flash movie) and Cathy Yan (who they hired because they liked her pitch).
If this were a vanity project for Robbie I would think she would have done the solo Harley movie that WB wanted her to do but she thinks Harley works better surrounded by other people so she insisted it be a group film.
No one who had seen the movie have been offended by any gay stereotypes. That's coming from people who haven't seen the film and have heard that Black Mask is "obviously" gay. They're just making assumptions. If you look at any of the women in charge of this film or any of the people starring in the film, I think it's fairly safe to assume that they won't be perpetuating offensive stereotypes. That's not too say that Mask may not be flamboyant, but some gay people are. Plus he's not the only gay person in the film so they'll be showing more than one side to homosexuality either way.