Stability is a key. We are working on that. I really want to put many features into core, but there is a negative side too that effects stability of core if done too fast.
Leaving some room for developers to create extensions

Please check that FTP does put all files correctly and did not fail in the middle.

USA Location zone that is set up in default install is set only to have New York state. Your shipping address is not in NY.

I tested with NY address and works perfectly.

Locations/Zones are set specific to the online store business and operations.

To edit Locations go to System -> Localizations -> Locations

I am sorry for the confusion.

In next version we will set all state to the default USA zone.

Something happened to your /core/init.php and definition of DIR_SYSTEM is no longer present.

Check if you have following line on /core/init.php line 274

define('DIR_SYSTEM',       DIR_ROOT . '/system/');

You can try to replace this file /core/init.php completely with file provided in upgrade package in /code/core/init.php

Possibly file was not copied completely.

It is simply uploading file over (replacing) and running SQL. That is exactly what happening under the hood, when you do the upgrade in the admin. 

Unfortunately, we do not have instructions for manual upgrade at this time. It is good point to have it and we will create it. 

Yes it can be used, but we migrate to GitHub, so better to use.

https://github.com/abantecart/abantecart-src/issues

Thank you for suggestion. We will consider this. Unfortunately, I can not give you exact date yet.

Excellent point about security. We will give it more thinking.