The teaching criteria gets more difficult as time goes on. And as you might expect, the lessons you do later in the course have a greater weight when it comes to deciding whether you pass and what grade you get. So yes, you can mess up a couple of lessons, just as long as they're not your final two lessons.
I don't really understand what you mean by how long the lesson plan should be. It will be as long as it needs to be based on the number of tasks you plan in the lesson, surely? You also have to do a language analysis as part of the plan after your first couple of lessons.