It is better to show qty available during invoice line, when we select the item. If you have too many items, it is difficult to move around screens and not practical.
That would be nice but not required, if there is insufficient stock then Back Order will be displayed under Inventory Items.
This is required, because most of the business owners requesting this, so at the point of selling they can check the stock level, if you have few stocks not a problem, but 1000 of items not possible to go back and check the back order. This is to check the availability at the point of sales.