1) Standby control file inherits attributes from the Primary database. What makes the standby control file separate from the primary control file?
Both are same. since standby database resides on other server so both primary and standby database cannot share same control file.
2) Does Standby Archival operation depends on primary database archival operation?
No. You mean if primary database is in archived mode then standby alos will be archived mode. Your standby database may or may not be in archived mode since there is one to one DR you have established. Your primary database must be in Archived log mode otherwise what is the use of creating DR.
3) A primary DB can maintain lot of standby databases. So DB_UNIQUE_NAME init parameter seperates which standby is maintained for LOG_ARCHIVE_DEST_n. Am I correct?
Yes. Every standby database must have different db_unique_name otherwise you someone will be in mess.