Re: Does " less established in the discourse" mean "not well accepted in spoken langu
No, not at all.
It means that so far in the conversation (in the discourse) we know more about the person or thing doing something (the agent) than we do about the thing that received the action (the subject in a passive sentence).
The passive is a "stylistic train wreck" almost every time you try to use a pronoun. To use a pronoun, you have to know who or what the pronoun is referring to (its antecedent). For you to know who or what the pronoun is referring to means that the antecedent has been "established in our discourse." You should not know more about the "who or what that is doing the action" than you should about the subject of the sentence if you want to use the passive.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.