Hi @OvidiuS
I'll run through each point and try and help you.
This is possible. When you convert your estimate, you would be given the option to 'tweak it'.
If you do it through the overview screen, clicking this icon:
Allows you to create a single invoice, and modify it before editing:
If you do it through the estimate view screen, clicking the convert button will give you the option to tweak it first:
From this screen, you can add, remove lines and details as you would on a normal invoice.
You could copy estimates before converting them. You can also copy invoices in the same way:
I hope that helps!