[moravak1984]

> 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?

[gora_kh]

Paper on this subject:

https://www.google.com/url?q=https://www.academia.edu/491578...

Evidence for "vuestra merced":

http://elies.rediris.es/elies22/cap7.htm

Much more in-depth treatment of "vuestra merced" etymology:

www.academia.edu/39806097/_Fue_vulgar_y_plebeyo_el_origen_de_usted_La_diacron%25C3%25ADa_del_pronombre_de_respeto_desde_la_interfaz_oral_escrito&sa=U&ved=2ahUKEwjc_-qd9tuEAxXFOjQIHWgZDMsQFnoECAYQAg&usg=AOvVaw1A_5QRn5k5EbDqDOzJ-DUy

Spanish Corpus:

https://www.corpusdelespanol.org/hist-gen/

Interesting review of dialects of Judeo-Spanish from 1894 where "su merced", "su osted", and "usted" all appear:

https://www.cervantesvirtual.com/obra-visor/la-lengua-y-la-l...