Finally got the issue resolved on my own. Rockstar gave me lots of suggestions, none of which worked, and ultimately told me the problem was likely due to my older OS (Win7) and strongly alluded the current version of the game won't work with Win7 and would require upgrading to Windows 10. I didn't feel their assessment was accurate but by that point Rockstar had run out of ideas and pretty much dismissed me.
I eventually discovered the remedy on my own through process-of-elimination. I deleted my Profiles folder in My Documents/Rockstar Games/GTA V and the game started right away, albeit at the very beginning. It did NOT require an upgrade to Windows 10, as I'm still using Win7.
I save every new game version and simply loaded a recent save, which I archived prior to experiencing any problems and was able to go right back to where I was nearly two weeks ago when all this began.
The problem has been RESOLVED.