Well, I managed to sort this out myself - thanks to anyone who read it, and put some thought into it, though!
LOL........gotta love/hate this game, right?!?! It definitely was a simple fix - I stumbled across another video on YouTube posted a few years ago. And, it was something obvious. Best advice - always backtrack through whatever you have done lately!
As part of working through the original "unable to launch, try reinstall" issue that many were having - which turned out to be that args.txt file - one of the possible issues I read about/saw in a different YouTube video, was changing compatability settings. In the both the GTAV.exe, and GTAVLauncher.exe files - the suggested fix was to check off "Run as Administrator". I did that, no help - until seeing all the args.txt comments here.
But, apparently the game ultimately did not like that! The more recent video I watched for my specific issue now, the "unable to launch GTA5.exe" error, mentioned the same file compatability checks.......but to make sure those WERE NOT checked off. I unchecked them.......
Sure enough.........problem fixed! Maybe this weill help someone else down the road.
Man....this game!