Here's some info on row level security - http://onlinehelp.tableau.com/current/pro/desktop/en-us/help.htm#publish_userfilters.html
Tableau won't be able to handle the offline scenario for you, you would need to code that yourself. After all, if the web page is trying to connect to the Tableau Server and it cannot make the connection, Tableau can't help.
You would need to build some some of a caching / PDF mechanism to display if and when the app goes offline.
Please discuss with Tableau Support / your account manager the licensing implications of embedding a report for many users which may or may not be users of Tableau Server.