I unfortunately cannot give input on how it handles corrupted files, but what I will say is:
The 85.9GB folder I have on my external worked after copying the GTAV folder onto my secondary drive; launched Social Club and no update or download was required.
However, when I did the same method yesterday for the RGL, it has to down a little over 2.something GBs extra… not sure why.
As for corrupted files - this is something you/we will have to discover.
with this whole Launcher thing im so keen to play some GTA again but yesterday was hard and i remember why I could never complete the Heist because people leave if you die ><
Also - the RGL appears to be more reliable than SC ever was. Loading into the app then running the game is much smoother and faster. The SC Launcher was just terrible.