I think we have to assume there is a camera covering Short B, whether it's the rear Cornhill cam or a private one. And it must show at least figures walking past even if shadowy / blurry - the fact Corrie was in white jeans would mean he could be ruled out quickly but perhaps the images weren't good enough for identification of others, most of whom would be dressed in darker clothes.
It seems like it's been tested by the family and the police are in agreement, and it's pretty much the only constant we've been given. Surely they couldn't ALL be so wrong?