How to Fix Disk Utility Getting Stuck on "Verifying storage system"
So I tried running disk verification with the volume unmounted and verification occurred without getting stuck as before.

You can unmount a volume from Disk Utility (don't eject it), but if it's your system volume then you'll need to boot into the recovery partition (hold Command + R during start-up) as the volume won't unmount otherwise.

I still can't determine why verification is getting stuck for the volume while it's mounted, or why Disk Utility isn't unmounting the volume automatically, but all tests are returning success so at least the file-system appears to be okay.