I believe that stendy is fairly toxic because they've broken up multiple times and wendy kissed cartman and they're always fighting but that's mostly because the boys in the class hate the girls and the girls in the class hate the boys for the most part.
Parker noted in the episode's commentary that the crew at South Park Studios began getting nervous, as a day's worth of animation work had been lost.
Why does Kenny never die? They can reintroduce Patty and make her a female parallel to Cartman.