Any caches that Alfresco uses are emptied / cleared when Alfresco is restarted. There is no intended way to clear them at runtime - technically it is possible but you'd be doing this at the risk of breaking your runtime environment (some caches are not meant to be cleared as they hold singletons).
You don't need to do any SQL magic to set an admin password. Simply use either the UI to change it or pre-define it via a configuration in alfresco-global.properties
The only thing you need to undeploy the process definition is the workflow console, correct, But your workflow model will remain active, any nodes / metadata created or set based on a type or aspect in that model will remain in the database.
Generally you should NEVER have to work with the database directly. It is strongly discouraged and not supported by Alfresco in any way.
