Hi getnami,

1. Export is NOT backup!!
Backup is copy of database files made by OS utilities, SQLPlus or RMAN.

And the answer on your question is NO because you cannot apply redo to an imported database - archive log files are useless in that case.

2. & 3.
You should take RMAN backup for both database files and archive log files. You need old archive log files for point in time recovery - but if you are struggling with insufficient disk space for backups you could delete old archive log files after making full backup.

My advice would be to backup archive log files to another storage and delete them from /arc partition on daily basis. It is very important to avoid filling this /arc partition. You should monitor disk usage on daily basis for that mount.