I suppose this is subjective, but I think that 写 and 号 are visually similar.
Don’t worry there will be worse (like 土 and 士 for example)
Yes. Wanikani acknowledges those as visually similar, but not 写 and 号.
The probably weren’t added by the staff simply because they share no common elements, though yeah, if you kind of zoom out they have roughly similar profiles.
Not sure if they’ll stumble on this thread though, so your best bet is to email them at firstname.lastname@example.org