Probably the best you can do at this point is to have the students submit a hardcopy of whatever they would like to count as their "final revised" version and weigh that version as the majority of the grade. They can try to edit the pages on the live wiki and give you a time-stamped revision to grade, but you could get some disgruntled wiki editors on your hands if your students start reverting changes outsiders have made because their student comments told them to do so.
Grading the "rough" drafts is just going to be a mess unless you gave them specific instructions not to post anything until they were ready to call that their first graded draft. Some students may have posted works in progress in good faith (i.e. not realizing that ANY new page comes up on a "recently added/changed" page search) and therefore (inadvertently or otherwise) have gotten more outside input than their peers.
Grading the "rough" drafts is just going to be a mess unless you gave them specific instructions not to post anything until they were ready to call that their first graded draft. Some students may have posted works in progress in good faith (i.e. not realizing that ANY new page comes up on a "recently added/changed" page search) and therefore (inadvertently or otherwise) have gotten more outside input than their peers.