> has been demonstrated in written and printed matter across the centuries.
which centuries are we talking about?
> It helps that written Spanish has been particularly well documented from the Age of Discovery onwards.
and that was how many centuries after the Arab occupation?
There are tons of Arabic-origin words in Spanish, (alcoba, aljibe, almohada, cerveza, naranja, zanahoria), so the hypothesis of "usted" sharing said origin is plausible.
Maybe you have some relevant evidence to your theory?
