For Item 1, you can set a portal to "slide up, also resize enclosing part" in the Inspector and then the unused rows will not appear when you preview or print your report. A much more flexible approach is to use a list view layout based on your portal's table instead of the purchase order table. The Invoices starter solution that comes with filemaker can be examined for an example of how to set up and use such a layout.
A simple invoicing demo file created by Comment may be even easier to learn from:
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