In order to sort sale.order.line, you need to have something you can use to sort. You probably want to add a new field, which you fill when saving/modifying the sale.order.line.

This requires you (in global wordings):

• to create your own module, which extends sale.order.line, and add the new sort field
• change the default sort order
• override create
• override write

Also:

• create a module which extends sale.order
• at break of sort of sale.order.line, add a line subtotal

I probably have forgotten some things, but I hope you get the general idea.