Your argument in general i agree with, but there are instances when, irrespective of field, tenor and mode, certain grammar usage prevents comprehension, and thus they are MISTAKES!
I was sent to prison.
I was sent to a prison.
I was sent to the prison
I don’t play football now.
I’m not playing football now.
I forgot my bag.
I’ve forgotten my bag.
Saying any of these sentences ‘incorrectly’ can lead to miscomprehension. If the main objective of language is to communicate, when I fail to communicate myself effectively, I am making a mistake.
Under your principle, a dictionary is also just a guideline, or a dresscode, as no word carries any intrinsic meaning, it is all presupposed.
Yes, if you’re trying to communicate effectively, and you use a form that impedes your communication, then you are making a mistake. Nothing I’ve said contradicts this. And certainly it happens that there are no speakers (as far as I know) for whom e.g. “you and I between” comes naturally; so using this form will always impede communication. This is also in tune with my post.
A dictionary writer can do two things: they can describe how a language is used (which is an empirical question) and they can give advice to people on how best to use the language (which is more a matter of judgement). Either job may be done more or less well. But in either case, what you get is a description or a guideline. A language doesn’t consist of what’s in the dictionary; English is defined by how its speakers use it.
And no, of course no word carries an intrinsic meaning. But that’s not the same as saying that words don’t have meanings. A word like “milk”, for example, has a pretty clearly defined and similar meaning for all English speakers. But it’s in the heads and the utterances of the speakers that the word actually has meaning, not on the pages of the dictionary (except inasmuch as the dictionary counts as an utterance of its authors), which just attempt to describe how English speakers use it, or to suggest how they should.
There are a couple of further things worth noting. First, there clearly is a difference between ‘between you and I’ and e.g. ‘you I between and’. The second would seriously impede communication with any English speaker I’m aware of; so if you’re attempting to communicate clearly with English speakers, using that phrase would be a mistake. The first, on the other hand (the sentence the brothers Iggulden describe as ‘just wrong’), doesn’t seriously impede communication with any English speaker I’m aware of (though it obviously annoys a few of them), and is I believe the normal variant for several speakers of English. Since it can be advantageous to sound like the person you’re talking to, ‘between you and I’ may thus be preferable to ‘between you and me’ in some interactions.
But we have to bear something else in mind, which qualifies what I said about making a mistake by impeding communication: we don’t use language only to communicate (or curry favour). We also use it to assert our identity. And this (along with the other uses we put it to) may stop us from using the same form as the person we’re talking to, which may even impede communication. British people don’t necessarily start saying ‘sidewalk’ instead of ‘pavement’ when they’re talking to Americans, even though this may well aid communication. Again, there’s an obvious analogy with clothing to be made.
In general, an important point to bear in mind is that a language like English doesn’t exist as some Platonic entity that its speakers know more or less well. Inasmuch as there is a coherent entity called English, it is a statistical abstraction: a clustering of speakers. And here the obvious analogy is with a species: the Polar Bear species doesn’t exist beyond polar bears; it’s an abstraction over them. It is derived from them, not the other way round. And the same relation holds between a language and its speakers—or, more accurately between a language ‘as a whole’ and the language in the speakers’ heads (their grammars) and on their tongues (their utterances).