And here's the latest:
I am now able to boot an APFS volume (on 10.13.6) with the original files backed up (from the HFS+ volume) and restored to the new volume. However, Clover Configurator can't see it, and disk utility reports the following (see below - size errors). I always thought Disk Utility's "First Aid" was useless and now I'm sure, because First Aid reports no issues with the Volume.....any of the volumes and I can't see the EFI partition no matter what I do. I have to remove the Micron SSD and load it on my X99 to edit the EFI (which, on the X99 is easy to see and access), and there, on the X99 (which is now APFS) it reports no sizing error in disk utility.
@elfcake: Yes, I'm beginning to believe there is something wrong with the hardware, because two months ago I attempted an install of Windows 10 on this hardware (for dual boot purposes) using a Crucial MX300 and it would not work, no matter what I did. I discounted it because a different SSD worked fine. But now I'm not so sure. Still, it's strange that APFS itself creates the problem, and HFS+ seems fine.
On the Z370 (10.13.6 - APFS):
View attachment 341549
On the X99 (10.13.5 - APFS):
View attachment 341553