How to find all consistent SCNs or SCN range that I can recover database to ?Not sure if we give the same meaning to "consistent", but all SCNs are "consistent" : see this AskTom thread, where you can read
can recover the backup to any SCN forward and this SCN is not required to be v$archived_log.FIRST_CHANGE#, i.e. checkpoint SCN ?Checkpoint SCN is required for cache recovery in case of instance crash. It is irrelevant for media recovery - any SCN you recover database to will be checkpoint SCN. You only need archived logs that contains this SCN from all threads.