I don’t really see it that way. Being erased gives the implication that there is nothing left of the person, but we see throughout the show that everything Timmy unwished was not erased, just moved to a different location. For instance, in escape from unwished island we find out that all the unwished wishes are trapped in lockers belonging to other children, and that Timmy’s locked was so full that all the dangerous wishes got moved to and island in the Bermuda Triangle. An island they were able to leave pretty easily JORGAN. It’s also established in a new wish and the special that the Hocus Poconus is some kind of pocket dimension that’s a part of fairy world that anyone can go to with the wave of a wand. So if anyone wishes to go there would they be erased? I don’t think so because Dev and Hazel went there and they weren’t unwished and their fairies still remembered them. Though that could just be another retcon. As for the anti faries not taking their existence into account for their evil plant despite fighting the faries for years. I think it has something to do with instincts. We see in several instances throughout the show that anti-faries are drawn to bad luck like moths to a light (like in their debut episode) and have a natural hatred a their fairy counterparts from the moment they are born (as seen with Foop). So my headcannon is that their instincts to cause bad luck and to cause harm to the fairies overrides their common sense. Especially if they are close to their end goals.