Hmmm. You’ve hit on all the points I’d want to check, especially with wow64 redirection, relocating the powershell script…
Let’s try and see if it’s a problem with using the LocalSystem account. You can emulate the environment in which Bigfix runs, using psexec ( https://microsoft.com/sysinternals )
Try running
Psexec -i -s cmd.exe
That will give a command promot runnin as the system account. Try running the powershell script from there and see whether it’s successfule, or whether it gives any useful error messages.