I am unclear if the students are continuing on as a group or as individuals after Lesson 13. When they link their screens it seems that they are now independent of the group because they can make their own changes. It seems that testing and improving would be more efficient if they stayed together. Also it would minimize the number of app presentations. But the lessons do not seem to give any guidance about how to manage them once they share out their screens.
From what I can see, these lessons are still together as a group, but also include students working independently. In Lesson 14, students are working as a team to gather data from potential users of the app. Lesson 15 has them working independently to correct anything that came up in their testing in Lesson 14. This seems more like a divide and conquer approach while 16 is altogether.
Maybe someone with more experience with this unit might want to share his/her ideas.