I agree with the previous posts--fascinating discussion, really! I would go with Tessie as main character, however. She does change throughout the story, and I find it particularly moving that her son is the first to cast stones at her.
While the box and the lottery are great candidates for main character (and they do both dominate the actions and reactions), they do not grow or change in any way. At the end of the story, the black box is still ragged, faded, and neglected from years of use. The lottery, despite the "talk" of rejection or alteration from the townspeople, does not change with Tessie's death. We as readers expect it to be back in full force with the next spring crop.