Most disk implements the so called "surface area scan", which runs automatically and periodically. This kind of scan happens on the entire disk surface, even on empty/free areas. On the other hand, a btrfs scrub only checks used space, meaning that empty disk areas are not checked.
This means SMART found some issues on unused sectors, but they are ...
Shodanshok's answer is excellent, but to answer your literal question:
What would be the best way to verify the health of the disk?
Do a full write on it. The disk's firmware has marked those sectors as pending reallocation. It can do this when they're written to. This will either 'fix' the sector, or generate reallocated sectors, which you can also see in ...
In addition to resizing the LV on the host, tell the running guest about it: virsh blockresize <domain> vda
As a paravirtualized driver, virtio does not pretend to be SCSI anymore, rescan hints will not work. As soon as the guest is aware if it, the volume will be larger.
While not useful in this case, a more complete SCSI scan script for Linux is ...
Synchronous writing mode ensures that the writes end up in a persistent location immediately. With asynchronous writes, data is cached in RAM and the write call finishes right away. The filesystem will schedule the actual writes to final location (hard disk).
In ZFS case, the point of ZIL / SLOG is to act as a fast interim persistent storage, that allows ...
I understand this comes with the risk of data loss if the drive fails
or power cuts out. This is acceptable as I retain the files on the
source machine long enough to retransfer in case of near term data
If you can tolerate the loss of up to 5 seconds of writes, you can simply configure ZFS to ignore sync requests with the command zfs set sync=disabled ...
Those SAS cables run four SAS lanes. Whether they do so in one, two or four cables doesn't matter, that's merely cosmetic.
You should check the rated maximum speed though if your intend to use the cable for SAS-3 (12 Gbit/s) or possibly SAS-4 (22.5 Gbit/s).