Hi rubyweb
I will answer you both questions:
First is, is the multi-inventory plugin capable of displaying the different between stock that is in the shop, and in the warehouse and further to that, is there an ability to only sell stock online from the shop inventory only?
-I'm not sure exactly what you say with 'the different between stock that is in the shop, and in the warehouse', in any case if you create a multi-inventory product, you can create 2 inventories, one for the stock in shop and another for the stock in warehouse, and you will see clearly how many items have each stock.
Regarding the possibility to only sell the shop inventory, I recommend you to do this.
Create the multi-inventory product with the two inventories:
Set this option. In this way, when the Main inventory will be out stock, the product won't be available anymore:
You can also set these settings for all your products (Multi-Inventory settings):
Second question is because there are multiple shop websites, would we be required to purchase the plugin for each website, or is it a multisite key as long as we own all the websites?
ATUM works fine on a WordPress MU but every site has its own data + config and the data can't be shared between them (at least not by default).
Best Regards,
Andrea.