Grading criteria changed mid-semester. Not fully communicated. What can I do?
You can try, of course, the question of who to depends on your university. I would start with a student advisor for your subject and take it from there. If you combine your efforts with the other students who also have trouble because of that change right now, that might help.
Furthermore, you should not start with officially appealing right away, as this might force people to take a defensive position against you. I would rather explain the situation, the problems that it causes you, etc. and ask the advisor for a solution.
If you formally appeal and try to get a passing grade through that, that might or might not work. But it will surely burn some bridges and, even worse for you, the whole process will take even more time than writing the missing chapter would.