Oracle RMAN Incremental update or merge questions
1: Incremental image copy update is only supported on disk

2: Updating the image copy is done using a differential backupset.  So you need the diff before you can update the image copy.  A good description of the process is in the Oracle docs

For both answers, see the RMAN Backup and Recovery User's Guide: http://docs.oracle.com/cd/E11882_01/backup.112/e10642/rcmbckba.htm#BRADV8186

Table 9-2 explains the process in detail.