OK, the issue appears to be with GPSMapEdit (I am using v2.0).
I found another windows program that can compile *.mp files into IMG files, which is primarily what I was using GPSMapEdit to do.
I simply substituted the new software IMG file in place of the GPSMapEdit IMG file, used MSTK again to install the map into MapSource, and MapSource opens and allows me to view my map without issue!
I sent the map from MapSource to my Montana 650 for verification, and some segments are missing from my map, although they are visible in MapSource, and some others are MIA from both MapSource and the Montana.

SO my convoluted workflow is now:
Use GM to search for and extract specific data types within *.shp files, and export as GPX.
Use GPSMapEdit to import, catalog and sort/align GPX files, exporting final product as *.mp.
Use MapTK to compile IMG from *.mp
Use MSTK to install IMG into MapSource
Still having other issue with MapTK software, however, so I would very much like to determine how I can stop the Map Key issue as described.....