Deleting headers and footers in Numbers

You cannot.

The header and footer can be blank, and you can size them to be really small, but you cannot delete them entirely.

The closest you can get is File -> Print -> Print Setup (in the Inspector) where you can adjust the margins and header/footer offsets:

But they will still exist.

This question comes up a lot. Might be worth asking Apple for an option to remove them entirely -> Feedback - Numbers - Apple

> the print task still seems to require a second (blank) sheet

That's certainly not normal. most of the sheets I generate are small, easily fit on one page, and I don't have a rogue extra page when I print.

Is there something else on the sheet that could be extending the boundary of what's to print? some shape? text object? etc.