I see you are trying to avoid using portals in your layout. That's a good idea, but in this case you may want to try using them for your sub line items and base your report on the line item table. You can make the portal many rows tall and set it to slide up|resize enclosing part.
The other option I can think off requires adding a "dummy" sub line item record for all lineitems that do not already have at least one sub line-item. Such a record could have all fields blank on this layout and the fields can be set to slide up/resize. That's possible, but seems messier to me than using a slide/resize portal here.