Honestly I don't know what to do if you failed in essay
I guess there are some other friends also asked the same question in other topics here. Please do a search.
For the diagrams,
Cades book is a good reference. But what I did was, I googled for each diagrams and read small articles from IBM web site and javaworld.com. Wiki also was a good source.
Once I gone through these sites I got ideas on how to go with the diagrams. So basically Cades book helped me to understand what is expected in the exam.
I also didn't put helper classes and other java beans in the class diagram. I just mentioned in notes that I am not including those in class diagram to control size.
So My class diagram contains,
1) Main JSPs
2) The controller class. (Servlet)
Similarly I tried to include the required information only in all the other diagrams. For example I didn't put login sequences in the sequence diagram. I just put seq diagrams for all use cases and mentioned in notes that login sequence is not included to control size.
Basically whatever you follow, you need to make sure that your diagrams express your ideas and patterns. Thats all.
Let me know if you need anything more...
Also use a good tool to draw the diagrams. I used 'astah' tool and is excellent one.