Convert a secure string to plain text

You are close, but the parameter you pass to SecureStringToBSTR must be a SecureString. You appear to be passing the result of ConvertFrom-SecureString, which is an encrypted standard string. So call ConvertTo-SecureString on this before passing to SecureStringToBSTR.

$SecurePassword = ConvertTo-SecureString $PlainPassword -AsPlainText -Force
$BSTR = [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($SecurePassword)
$UnsecurePassword = [System.Runtime.InteropServices.Marshal]::PtrToStringAuto($BSTR)
[Runtime.InteropServices.Marshal]::ZeroFreeBSTR($BSTR)