1. Should Class Diagram have ONLY business domain classes or other classes like Business Delegate, Business Facade, ServiceLocater, other helper classes etc?
2. What level of details should the classes have as far as attributes and methods are concerned? should datatype of attributes be shown? should signature of the methods be shown? I know, the Exam instruction does say that we do not have to show everything. but should we not show at least all the methods that are used in sequence diagram to realize the use case?
I think I can give you some advice as I just certified to be SCEA recently.
For the class diagram, only domain classes and service classes (which I modelled as session bean) are sufficed. To get a higher mark in this section, remember to scan through the requirement carefully and include those domain/service classes stated and have their relationships clearly be represented. No attributes and methods are defined for me (which is not the scope of arch. design).
For the component diagram, you may follow some J2EE arch. documented in Sun (actually some classic components like service locator, business delegate, session facade... are sufficed). You can divide the diagrams based on the use cases stated (auth/enquiry/reservation/payment etc..). Usually, I would draw a general component diagram to state out the overall design and all the components used first, and then list out the specific component diagrams (for different use cases).
For the sequence diagram, include the basic flows as well as some IMPORTANT alternative flows. It is not difficult to get full marks in this section if you have included all necessary sequence flows in this part.