We will update the manual soon.

With this feature, we allow to specify inventory from different locations. That is about it really.

I do not think, new stock location will not cover all possible use cases.
Many aspects here are related to order fulfillment process and logistics.
After all, we do not try to implement inventory management solution here.

We welcome suggestions and contributions into further development

New version of AbanteCart 1.2.14 is out.

Master branch is set to be 1.2.14.
https://github.com/abantecart/abantecart-src

New Features and improvements highlight:
https://github.com/abantecart/abantecart-src/releases/tag/1.2.14

I have see sites with over 100k of products with AbanteCart. AbanteCart does not have a limit and designed to handle large mount of products, customers, orders, etc.
A lot will depends on your web server configuration, products configuration and additional custom processes you might need to develop to support your data.

I assume you refer to form manager. Here is the manual for UI side.
https://abantecart.atlassian.net/wiki/spaces/AD/pages/13500486/Forms+Manager

If you need to have a special processing after form submit, you need to develop a custom controller to process your form and register it in the ac_forms table.

Here is a default form that we have that you can use as example:
INSERT INTO `ac_forms` VALUES (2,'ContactUsFrm','content/contact','content/contact/success',1);

Here is the controller example:
https://github.com/abantecart/abantecart-src/blob/master/public_html/storefront/controller/pages/content/contact.php

Your custom controller can be placed in any location including an extension. Make sure you provide correct path in the table insert.

There is an error somewhere in the JavaScript.

What version of AbanteCart do you have?
Did you do any modifications or installed any extensions?

To answer your why question, categories are collapsed into drop down for smaller screens. This is a feature for mobile devices and smaller screen resolution tablets.

We do not have under construction feature, but we have maintenance mode. You can change the test to "Under construction" if you want.

You can enable it in System → Settings → System.

See this manual:
https://abantecart.atlassian.net/wiki/spaces/AD/pages/7962661/Maintenance+Mode

Possibly, you do not have coupons enabled on the shopping cart. Check manuals below.

https://abantecart.atlassian.net/wiki/spaces/AD/pages/7143431/Coupon+on+shopping+cart+page

https://abantecart.atlassian.net/wiki/spaces/AD/pages/11304965/Manage+Coupons