Colombia's Special Word for "You"

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?

A useful aspect of the language that I was required to learn was a stricter focus on grammar. I got away for a while with a weaker understanding for verb conjugations during conversation, but the courses required me to learn this thoroughly. Though this wasn't necessary for general communication, this helped with preparation for a professional language assessment and for formal communications.

However, a less useful aspect that didn't matter much to my communication abilities came from taking a course on pronunciation. Though learning the pronunciation rules for the first two thirds was helpful for reducing my foreign accent, the last third required a couple assessments that heavily weighted students' abilities to identify the regional accent of a speaker (e.g. identifying between accents that were Andalusian versus Rioplatense versus Chilean). While there is arguably some value with identifying a speaker's accent from an interview or film, this took a lot of time that I could have spent learning other aspects of the language, and I wish this skill would've been optional to learn.

So, you've hit on one of the downsides of learning Spanish in a formal environment via inflexible expectations of learning goals, though I did find upsides too (that said, a motivated self-learner could absolutely learn grammar on their own—though a classroom environment does provide a nice motivation).