1)Do i have to necessarily put the lun offline before unmapping ? If yes , I do I have to put it online again before running step 3 . i.e when running snapmirror resync. Or the step mentioned above is good .
2)Also, I do not see the flag to unmap the luns using lun_id in step to 2 . For example , I used lun id 60 to map the luns , do I have to specifically mention the -lun-id when unmounting
No you don't have to "offline" a lun before unmapping it. You can just unmap it from the igroup containing the hosts, then Rescan for Datastores on the hosts/cluster and it should remove all new dead paths.
And No you don't have to specify the LUN ID during unmapping it. You only need to specify it during "mapping" if you want it to be a specific LUN ID and not just the next one available. I always make sure to use Specific LUN IDs and keep track of them on a spreadsheet.