Dear ustas
first of all, please open a new thread with one question only.
Anyhow:
1. REST API are documented in Apiary . The document content subresource is conceived for documents execution, but it is available only for static reports (JasperReports and Birt reports) at the moment.
2. When executing a document, if user's role is enabled (check role authorizations), he can get the direct link to the document (execution toolbar --> Copy Link functionality). In case the document is public (i.e. executable by a role that is set to be public), then no authentication is required, otherwise user has to authenticate.
For SSO, you can use CAS (documentation here), or even other systems, but a custom implementation is required in that case. The API to be implemeted are described in SpagoBI wiki, check these links:
http://wiki.spagobi.org/xwiki/bin/view/spagobi_server/User+Management_v3#HConnectors
http://wiki.spagobi.org/xwiki/bin/view/spagobi_server/sso#HHowtoenableSSO
Hope this helps