New drive arrived. 'zpool replace' worked without a hitch, even if the old drive was showing as FAULTED instead of OFFLINE.
For reference, this is what worked for me:
/etc/init.d/zfs-zed stop
zpool replace data ata-ST12000NM0007-2A1101_ZJV2LDGN /dev/disk/by-id/ata-ST12000NM0007-2A1101_ZJV1T4YF
/etc/init.d/zfs-zed start
In summary: Just chill, everything will work out fine. 
Shortly after setting up my zpool, one Seagate drive started showing bad sectors ... so it is being returned for a replacement.
The procedure for replacing the drive, seems to be:
Unfortunately, zpool offline does not have the expected result of changing the state to 'OFFLINE' -- the drive still shows up as 'FAULTED'.
root@omv-nas:~# zpool status
pool: data
state: DEGRADED
status: One or more devices are faulted in response to persistent errors.
Sufficient replicas exist for the pool to continue functioning in a
degraded state.
action: Replace the faulted device, or use 'zpool clear' to mark the device
repaired.
scan: none requested
config:
NAME STATE READ WRITE CKSUM
data DEGRADED 0 0 0
mirror-0 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV310R4 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV3B4T7 ONLINE 0 0 0
mirror-1 DEGRADED 0 0 0
ata-ST12000NM0007-2A1101_ZJV24C09 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV2LDGN FAULTED 0 0 0 external device fault
mirror-2 ONLINE 0 0 0
ata-WDC_WD80EFZX-68UW8N0_VK1B10SY ONLINE 0 0 0
ata-WDC_WD80EFZX-68UW8N0_VK1E4SPY ONLINE 0 0 0
mirror-3 ONLINE 0 0 0
ata-WDC_WD80EFZX-68UW8N0_VK1E696Y ONLINE 0 0 0
ata-WDC_WD80EFZX-68UW8N0_VLH2U7GY ONLINE 0 0 0
errors: No known data errors
I have tried all the variations of 'zpool offline' I could think of:
There are no errors reported from the 'zpool offline' commands, but also no change in the state of the drive.
Will this cause me trouble when the time comes to 'zpool replace' this drive with a new drive or should I just chill?
Thanks for any insights or shared experiences!
@subzero79 Thanks a lot -- nice to know that the issue will be looked into!
For now, I decided to create the pool using CLI and ignore the ZFS GUI until the bug is fixed. Filesystem shows up fine in OMV -> Filesystems, after all.
Thank you for responding!
After rebooting, the pool is still missing from the ZFS overview. Also, the message 'A mirror must contain at least 2 disks' immediately pops up when selecting ZFS. zpool status still seems OK.
Is the ZFS GUI unable to handle a pool with several mirror vdevs? Maybe I need to use a couple of RAIDZ2 instead (4x12 TB + 4x8TB).
Edit: Creating the pool by CLI and rebooting: Same result. No pool in ZFS GUI, error message pops up, zpool status looks OK.
Fresh install of OMV5 with proxmox kernel (1) and openmediavault-zfs (2) from omv-extras.org. Trying to create a ZFS pool with four mirrored vdevs (2x12 TB + 2x12 TB + 2x8 TB + 2x8 TB).
I'm able to create the pool with the first mirror vdev, but expanding the pool with second set of mirror drives fails. Error message: A mirror must contain at least 2 disks. Yes, I am selecting two drives when expanding the pool
Confusingly, zpool status seems to be OK?
root@omv-nas:~# zpool status
pool: zfs-tank
state: ONLINE
scan: none requested
config:
NAME STATE READ WRITE CKSUM
zfs-tank ONLINE 0 0 0
mirror-0 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV310R4 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV3B4T7 ONLINE 0 0 0
mirror-1 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV24C09 ONLINE 0 0 0
ata-ST12000NM0007-2A1101_ZJV2LDGN ONLINE 0 0 0
After the attempt to expand with a second set of drives, the pool is missing from the ZFS overview. Also, the message 'A mirror must contain at least 2 disks' immediately pops up when selecting ZFS.
Should I expand the pool using CLI instead of GUI, or what am I doing wrong here?
Thanks for any pointers in the right direction!
(1) Proxmox kernel installed because the 5.2 kernel in backports is currently uninstallable: linux-image-amd64 depends on linux-image-5.2.0-0.bpo.3-amd64 which is missing from the backports repository.
(2) openmediavault-zfs installation initially failed because zfs module wasn't ready when setting up zfsutils-linux. Fixed with modprobe zfs; apt install.