This seems like a lesson that will elicit powerful emotional responses from students. Although the site is rather disturbing, it gives history and talks about conditions that cannot be forgotten. I can see how the lesson would draw students in and make them eager to give their opinions in the letters. From a Universal Design for Learning perspective, it is good that you included graphic organizers and sample writing to help guide students in their thinking and what is expected of them. Having the visual pictures available to support student learning also is important. It will make their learning more memorable for this particular assignment.
Thank you for including the link to the website. I did not find any technical problems.