Thank you for your answer, Rusty.
I've taken a deeper look into these rules, checked much more sites and most of my questions are now answered. My first one though still remains
una duda. Most of the sites says that the nouns ending in tonic "á" or "ó" add "-es", and those which add "-s" are exceptions.
Diccionario panhispánico de dudas says exactly the opposite [para b].
Can a native speaker clarify how it is nowadays?
Also (this is pure curiosity), are there in Spanish oxytone words ending in "x"?
Thanks!