With respect to the question about how many instances of GIMP I'm running, the answer is I try to stay up-to-date and mostly use the latest version. There are 2 primary reasons for preferring my portable installation method. One is you can install it once then take it to other computers. I have several but I could also take it to other peoples computers which would be a rare occurrence. Second, it is quite normal for new versions to exhibit new problems NOT previously experienced. It is very desirable to only have to close the one causing problems and start a prior version that works correctly. I started doing this more than 10 years ago and have just about every version of GIMP starting with 2.8 available. Should disk space become a problem I'd probably be happy to delete all but the last 3 or 4.
I've done some more review, including the referenced links, and I think there might be a simple answer to my question. It looks to me like there is a file named "gimprc". Apparently there are 2 instances of this file. One is located in the main folder where the GIMP package is installed called "GIMP 2/etc/gimp/2.0" which is considered a system wide version. Then there is another one intended for each individual user. It is NOT clear to me how this one is normally created. On the computer where GIMP was installed there is one in "C:/Users/AppData/Roaming/GIMP/2.10" but NOT for every user. My guess is that it may have been created by the Installer (i.e., for the user who performed the installation). There is No such file on either other computers (i.e., other than the computer used to install GIMP) or for other users (i.e., other than the user who installed GIMP on the computer where GIMP was installed).
As it turns out, I have been setting the environment variable called "GIMP2_Directory" in the scripts I use for starting GIMP which I think overrides or replaces the version of the file named "gimprc" which pertains to individual users. I have been sharing this file between different versions of GIMP which, I think, would explain how "Preferences" are being shared between my different instances of GIMP. Not having to reset preferences when installing a new version of GIMP is something that I find desirable and, at least up to now, has worked well. Insofar as I'm only guessing that it might contribute to
this problem that I'm experiencing with the GMIC Plugin I won't be making a change without knowing it is causing a problem.