Books,plays, films etc all operate within a heightened form of reality. Dialogue is crisper than real life, things happen in a sequence which serves a purpose, but is far from natural. Extraneous elements of real life are removed. All this creates coherence, which is the basis of storytelling.
Even film makers who get the cast to make it up as they go along, edit and select views which coordinate what would otherwise be incomprehensible. This reflects real life where when you are in a crowd at a party, you only pay attention to a small part of what is going on. The screenwriter, playwright and writer does exactly the same thing, focussing in on things he wants you to see to tell his story.
It is possible to put nuances into the spoken word (and the body gestures that accompany it) that can only be hinted at in written English. This is why some actors command higher salaries than others, their interpretations please the public.
Which brings us back to incoherence in writing. It is possible to write in patois and even invented slang (e.g. The Moon is a Harsh Mistress) and be understood and enjoyed. But this is a different matter from writing so that subjects and objects get confused within a sentence. The patios and slang in such books follow the basic rules of English comprehension. (At least in any book I've read)
Even the most incoherent teenager(or cat for that matter), walks, gets food, goes to places, opens doors etc in a coherent manner. If you use 1st person POV, then the internal dialogue between the reader and the writer must be similarly coherent and understandable. Because that's how it is in real life, let alone in the heightened reality of writing.
I raised a point on Authonomy about a sentence with too many clauses. We all write those in first draft because as the writer we never confuse the subjects and the objects in our sentences because we know. The ruby was in the grass, which glowed red. So what's glowing red? The ruby or the glass? This kind of sentence gives the reader a headache as they struggle to make sense of them. And it isn't even how people think. It's just a consequence of the translation between thought and words on paper.
I don't believe you can get away with an incoherent narrator. There is a place for too many clauses in dialogue, but not in narration, because in those cases, the writer is conveying inner thoughts, which are totally coherent to the character and therefore must also be coherent to the reader. They may not be explained, but they should always make sense eventually.