It is pretty obvious that the concept of ordinary language is theoretically void. If we understand ordinary language as "not poetic," then we have to have a concept of a language that is arhythmic, non-metaphorical / non-rhetorical... Where are we supposed to find this language?
We can talk about poetic language, but what we are doing is talking about language as it behaves in specific instances that we want to identity as poetic. That is not really different from talking about any other type of language usage.
Prose, as David Antin points out, is a system of notation. Since that's what it is it isn't a very useful concept theoretically unless we are talking about notation. Prose as "not verse" is a negative definition. In other words, we are defining writing by saying that it is not divided into lines of verse. Imagine if you notated music without dividing it into measures or having a time signature. Or you just wrote rubato. It would still be notated music, it just wouldn't have those cues.
Realism just doesn't exist. We have some literary conventions associated with concepts of realism, and with "realism" as a literary movement. But there is not such thing as representing reality accurately in a piece of literary writing. It just doesn't happen. Imagine documentary film-makers filmed 200 hours of footage in order to produce an hour-long documentary. So we get to see the half of a percent that tell the story that they want us to hear. There is no realism even in this genre.