This has been the most fascinating thread. Having four Shelties has given me some insight into what they might be thinking. If one wants to play, she'll snap and bite the other. No harm -- it's all in good fun -- but Shelties don't think about the fact that their people don't have nice thick fur to cushion the bite. OUCH!!!
I suspect Ellie was just tying to play -- she had been good and restrained on the walk. The reward play is a wonderful idea -- think of how drug dogs get to play with the ball after they sniff a line of luggage.