So first, one does not have to conjoin two independent clauses.

By "have to" do we mean "to be comprehensible" or "to be [standard] grammatical?" And do you just mean that two independent clauses can simply be used consecutively, either with a semicolon or by starting a new sentence? If so, sure; I meant that if you're separating with a comma, you have to use a coordinating conjunction.

And while one can understand what is said in your first sentence, I am positive it does not prove your point. Here's why: You do not have two independent clauses in your first sentence. "I'm fairly convinced that your reading is" is actually a dependent clause. Since you do not clarify what the 'is' actually.. is, the meaning of the sentence is entirely dependent on what the clause before it justifies. I'm fairly certain that explaining the misunderstanding in your first sentence would be enough to exemplify the struggle throughout our grammatical debate; however, if you are not satisfied you are welcome to read on as I address your further points.

I'm not certain that's a complete account, because I think we're discussing an edge case. Typical dependent clauses cannot stand alone in a sentence in any case, i.e. "You like this discussion more than I do." Even if you expand the dependent clause here to become "You like this discussion more than I like this discussion," "than I like this discussion" cannot stand alone, obviously! In contrast, the issue here is merely incidental. For one thing, "I'm fairly convinced that your reading is" is grammatical, if relatively meaningless unless we're talking about its very existence. More importantly, if we apply a similar operation as before and simply expand the reference, "I'm fairly convinced that your reading is based on a misunderstanding" is perfectly grammatical. I'm not sure I remember the linguistic term, but it's essentially analogous to pronoun use. A sentence with a pronoun in it is not a dependent clause by merit of the fact that the antecedent lives in the prior sentence.

"Only" as a conjunction is standard English usage; however, still informal / can be construed as slang. This conception is what I addressed in my first post--in order for maximum clarity of the statement it would have been much better had the author written their intention instead of leaving it to the imaginative boundaries of vagueness.

So the reason I didn't interpret the point of your post as primarily conveying that this was the intended, but slangy, meaning was that you spent a lot of time working on this idea that there's a grammatical error and there should be a semicolon, which changes the meaning. I agree about clarity; I favor "except."

Furthermore, the examples from the OED are all fundamentally different than the Kelsey Moser sentence which started this all. All three of the examples listed offer a clear subject to which the pronoun after the 'only' is affixed.

^ I think this point also rests on our construction of what an independent clause is and if "only" can be used as a coordinating rather than subordinating conjunction