What you’re doing appears way too complicated. You don’t need to backup & restore the Powershell Execution Policy, because you’re bypassing that policy on the command line anyway.
Does the “C:\temp\WindowsUnqoute\WindowsUnqoute.ps1” file actually exist? Does it do anything when you run it on a command line? Does it need any additional parameters, for instance to specify a name of a file or whatever in which it is going to un-quote things? If the Powershell script itself works as-is (when run via psexec -i -s cmd.exe, as @itsmpro92 already noted), then you should be able to run the same via BigFix as simply
waithidden powershell.exe -ExecutionPolicy Bypass -File C:\temp\WindowsUnqoute\WindowsUnqoute.ps1