A lot of the cast is pretty terrible at times, but because we've been given a look into their thought processes and backstories we understand why. Yes, Daphne believing the rape story was terrible....but only because it completely fell apart when the timeline was checked. Sarah being her friend and Charlotte being generally terrible doesn't mean Charlotte is automatically a liar or that Sarah couldn't be a rapist. Most rapists are the last person you'd expect to be one, someone you've known for ages and see all the time. If the video had chosen a better time and hadn't implicated Penny as well there would have been a lot more uncertainty going on. And even despite the stupidity Daphne mostly just believed Sarah was secretly with Penny, something she'd continue to suspect for a couple years.
Think about it, how much worse would Helen have looked without the glimpses into her issues? What she did was pretty much right behind Cyndi, she helped everything that was going on, backstabbed all her friends, both new and old, because she wanted to feel like she was useful to someone she knew cared nothing for her. And finding out Charlottes deal really humanised her, both to the readers and some of the characters and even started helping her to recover.