Hi @Jabberwockie,
Sorry to hear this.
The usual suggestion for this would be to check your .NET Framework installation, and repair it - though I can see that you’ve already tried this.
Another reason this may occur, is that Anti-Virus software may be tampering with the launcher binaries/dependencies. Can you please try disabling any Anti-Virus software and see if this allows the setup to complete and game to launch?
Let me know how it goes!