Much appreciation and many thanks for describing and explaining your sequence of teaching ukemi. It's quite refreshing to hear that people are taking the time and energy to think about ukemi in such a systematic manner.
I also appreciate your emphasis on "principled cooperation," especially during the beginning phase. Additionally, I, too, believe that proper ukemi should be shown to beginners by the instructor in the same manner that the technique itself is shown.
Lastly, I think it's very telling that you have not had any injuries at a brand new dojo over its first two years. My compliments on the success of your dojo!