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...