@afre had the right idea from the start, it would seem.
Whereas I wasn’t able to use the installer version of GMIC to update to the new version without crashing, the .zip file solved the issue.
Removed “gmic-qt” folder from “C:\Program Files\GIMP-2.10.18\gimp\lib\gimp\2.0\plug-ins”, replaced it with the current 2.9.0 release and everything seems to be working smoothly again.
I’m still curious as to what may have caused this to begin with, because everything WAS working as intended when I first downloaded the pack and other people running the base pack don’t seem to be experiencing the same issue.
At least now there is some documentation of the issue and a solution (in my case) for anyone that might find themselves googling around for a fix like I was for a few days.
Thanks to David for troubleshooting with me, and thanks to afre for making us all look dumb and finding the solution. 