Is the られる used as a honorific (some sources use "polite", but I assume it's a honorific?) related to the passive form (perhaps it's alternative usage) or is it something that emerged separately? (would the difference have some potential impact?)
From what I found out:
"The passive is used: (...) as a form of respectful language"
"The honorific style can also be expressed with what is called the “easy keigo” with verbs used in the passive form れる or られる"
"Alternative Form: To Use the Passive Form"
"When using the passive form as a honorific form"
"using passive form is another more polite way to express an action"
Dictionary of Basic Japanese Grammar
"Passive verbs are also used as honorific expressions"
"Don't forget that there is a third conjugation of this form that is a type of keigo"
Author of Japanese from Zero in discord:
"That is not passive nor is it potential. It is a high level polite."
So no idea who is right. It being a different usage of the passive form would imho make sense as it would follow the politeness through indirectness approach (passive somewhat feels less direct to me) ... and maybe that it seems less likely there would be another conjugation to just happen to end with られる. But I'm not a linguist