1)Are Switchover & graceful switchover the same?I thinsk Graceful Switchover and Switchback is old terminology. Now change of databases roles is Switchover.
2)Does Oracle Data Guard support automatic switchover?Oracle Broker manged Data Guard configuration can support automatically failover. You can configure with observer FSFO - Fast Start Failover.
3)How long does it take to do a switchover?It is depend your applied redo to standby database, if your database runing Active database guard then switchover can seconds.
4)In order to perform switchover, does the standby database need to be in recovery mode?Yes. Redo apply started it mean, when you switchover to standby database, then stamdby database shall become primary and need standby recovered.