I am unable to reproduce the issue. With 1.1 installed I can run sudo ./uninstallMSC and it will uninstall correctly. I can also just run ./uninstallMSC and it will prompt for my credentials. I would say verify that the account you are using has administrative rights, but if sudo su is working for you it sounds like you are.
The uninstallMSC file is just a bash script; you might try working through it line by line in an administrative shell to see where it breaks and go from there.
Also, I have updated several machines to v1.2 without uninstalling v1.1 so that may be an option for you.
From what I've gathered from your post is that you're trying to uninstall the previous version of McAfee and install the latest one? One tool I've used in the past has been AppRemover (a free tool that is now available for Mac computers), it thoroughly removes any security software on a computer (works best for someone in your case that wants to remove an old antivirus and replace it with another). You can download it at www.appremover.com, hopefully this helps!