I really feel like a dummy.
I guess the problem has been fixed, but I found out that (at least with an MBR) all I need to have in the boot order is "Windows Boot Manager" then "HDD 0" instead of "HDD 0" being the only option. I thought that because I thought "Windows Boot Manager" only dealt with GPT disks. Oh well, now it's all back to the way it was once upon a time.
But. . .I still wonder why the error was occuring when I installed Windows on the GPT instead? And how much better (or what are the practical differences) in GPT vs. MBR when it comes to Windows on a UEFI friendly board?
Thanks again for the effort.