As far as I know you can't force the review to be closed by breaking the predefined rules (and I think that's a good thing), but as an administrator you have power to change things like review roles to ensure that the closure criteria is met. In the example you provided, if a critical reviewer left the company, and the review cannot be closed because they are the remaining critical approver you could change their role to an optional reviewer - if that was the last item to satisfy for the review then the review would automatically move to the completed phase.
