Questions about E-Commerce functionality

Base Kentico EMS functionality does not support 1, 2, 3, or 4 without customization.

HOWEVER, the integration with UCommerce does support 1, 2, 3, and 4.

Thank you Brenden. I'll take a look at UCommerce.

In case I need to make some of the custommizations myself, do you have any recommendations, e.g. how to store multiple tracking numbers and multiple inventory numbers? For example, is it better to customize the existing SKU and Order classes, or is it better to write a separate module and don't use build-in tracking number and inventory field in Kentico?

For tracking numbers, I was going to add a new field to the Order class, which can store a list of tracking numbers. For inventory, I'm thinking of writing a custom module instead of using the fields provided by Kentico.

I'd suggest creating a custom module with a custom class in it and writing to that class table during global events. This way you can get a reference to an order with multiple tracking numbers. Then create a UI for the orders which displays those tracking numbers for the order.

Regarding the inventory, that's a bit larger beast. Not saying it can't be done but essentially what you're tackling is rewriting an inventory module for an e-commerce solution.