So mintUpdate works, and you can add an icon per this post:
It's not available in the "add to panel ..." window, and dropping MintUpdate from the menu onto the panel adds the shield icon. Update still works fine, but I'd like the visual warning of available downloads.
So if you have the shield in the tray and you open MintUpdate, and select preferences and then icons what icons are displayed? If its the shield can you change the icon to what is located at: /usr/lib/linuxmint/mintUpdate/icons?