While I can't answer your question, I still don't understand why you are specially disabling avast service just to go back to Windows Normal Mode to uninstall it.
If you want to download the complete setup.exe to update, and for some reason you find that this method is working better for you, that's fine. The "problem" "could" be in the active avast service and defense module, but it may also be related to something else in your particular system.
But avast can be uninstalled from Windows Safe Mode, and the Removal utility
http://www.avast.com/uninstall-utility also works better from Windows Safe Mode.
In addition, you have not tried to delete the firewall rules of ZA, which is particularly known to have some conflicts with Avast.
So, I'm glad you simplified your update procedures for your specific systems, but keep in mind that,
A_ it can be simplified even a little more; and
B_ the normal built in (from Avast main GUI) update procedures are working correctly for other users.