Dear Vijay,
regarding your questions:
2. Existing assignment is not applicable for multiple users. This is only enabled for single users.
3. Does default parameters work for single user requests? Make sure you have not restricted for a request type. Also check if "User Defaults" action is defined in the used request type for multiple user requests.
4. This should definitely work with on your SP level. Make sure you have successfully ran all sync jobs in order to start the UAR workflow.
Cheers,
Alessandro