Hi,
The new system allow to edit only one part by one part the order.
And when editing some part, it refresh dynamically the other parts in AJAX. So you don't have to save, wait for the page reload to see the changes on, for example, the prices, or something else.
If you change the order status, you can check the box to send the notification email to the customer.
To add a custom message to this email, you have to click on the "mail" button in the toolbar of the order edition page.
And here you will be able to edit the contend of the email.