Indeed, the only way to assign multiple suppliers to the same product is our Multi-Inventory add-on as you've correctly stated.
Please, note that not all the users are using multiple inventories for multiple locations (this is only one of the ways you can use MI 😉 )
What is suggested to name the Inventory if I have one location and different suppliers?
Just name the inventories in the way you want. That's up to you. I would suggest you name them in a way that help you to understand quickly what kind of inventory is.
What meta-key I can use to add SKUs per product and multiple suppliers?
ATUM doesn't use meta keys practically. We use custom tables. So, if you want to edit in bulk any ATUM data (including multi-inventories), the only way is by using our ATUM Export PRO add-on.
Here you have an example of how to do this process:
How to combine it with the Product-Level Addon?
I'd kindly suggest you read the documentation as it's explained there:
I'm testing the demo now, but the drag-drop function isn't there anymore?
I'm sorry but I don't understand what you mean. Which drag and drop function, please?
Why the demo doesn't have the Manufacture menu option?
Which demo are you trying, please?
If you mean the "Manufacturing Central" table, this is part of the Product Levels add-on and can only be found on the Product Levels demo.
Is it possible to set the product SKU and regular/sale price globally within the multi inventories?
The SKU not, the prices yes.
For the prices, just make sure the "price per inventory" option is disabled for your products and the prices will be taken from the product and the inventories won't have fields for them.
This is also explained in the documentation:
After messing around, I've noticed that the field Supplier and Supplier's SKU is still available, after linking a product part to a variable product, which is already linked to a supplier.
What is its purpose?
Why not? If you create purchase orders and want to add BOM products to them, if they have a supplier assigned will be used for filtering the products that can be added to the PO if this has a supplier assigned too. Also, BOM products can be sellable and can also have multi-inventories. There are a lot of ways to use our add-ons and perhaps for some users some of the functionalities aren't needed but they can be very useful for others.
If you don't need these fields there, just leave them blank and don't use them 😉
I hope this helps.