Hello Tim,
About the first question, if the volume is NFS o CIFS (NAS), you can modify the size online with no impact on the RH 7.
About de second question, with the command "nfs connected-clients show" you can see which client and which volume is mounting. If the export is not in the fstab on the client, i should do a "df -h" or "mount -l" to be sure the volume is not mounted manually. If is all ok, you can unmount from NetApp. A good practice is go offline the volume and see if is all ok, and later delete it (be sure you have a backup if you needed).
Kind regards,
Albert