Total Weight = Empty box weight + Weight of all items in the return
Empty box weight: the weight you set in this very same dialog further down so make sure to double check the value you have set.
Item weight: the weight you have set for the variant in your Shopify store.
No, orders in Shopify are immutible and therefore the weight for line-items in existing orders is not updated. Changes to the weight of a variant is available to all new orders.
Yes, go to the Products page in your Shopify store and use the Bulk-Edit feature to change weights for multiple products at once. Copy & Paste also works in the table-like editor.