A little research reveals that Oracle (Sun) partner with COMSTAR for the iSCSI connectivity and this page from 2013 says that (at that time) scsi3 PGR was not supported.
It depends on whether your SAN controller supports SCSI3 reservation irrespective of whether you have SCSI3 disks in the array. There may be a manufacturer's firmware upgrade.
I've read your post a few times but find it difficult to determine exactly what's happening. However, without a doubt, although you might find it unavoidable to "abruptly shutdown" the cluster, for...