Answer to Question 1
An object has a set of attributes and these attributes accept a range of values. The combination of these attributes and their associated values constitute the state of an object: an infant boy grows to be a 80-year-old man; a sapling becomes a tree; a student becomes a graduate. While the identity of the object remains the same, its state might change.
Answer to Question 2
Use cases are a set of developed, structured use cases for the system or the subsystems that you are developing. It is likely that you have to revise the use cases later on, but this set must be good enough to sustain the next step of development: dynamic and structural modeling of the conceptual system. At least one diagram for the subsystem is necessary to summarize the uses cases and the relationships with the actors and each other. If a use case has too many extend and/or include relationships, you may create diagrams for the subset. But never, never feel that it is a good idea to create one diagram per use case. A use case diagram does not show what goes inside the use case.
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