I’m not sure that you need to leverage cmd.exe to call the batch file at the end of your script (which may be complicating things here…see Setting and using variable within same command line in Windows cmd.exe - Super User for potential info). Consider just running waithidden unintall.bat as the last line.
One other consideration here is that since this requires a password, I’d suggest leveraging the ‘TakeSecureFixletAction’ method to pass the value more securely. See the following for more information:
https://www.bigfix.me/fixlet/details/2593
https://www.ibm.com/developerworks/community/blogs/e9d21113-aa93-467e-ac77-a0d20a21eaec/entry/Secure_Parameters_in_Baseline_Actions_a_workaround?lang=en