I know that you are thoroughly aware that what is 'correct' in punctuation, spelling and even in definition is completely the result of common usage.
Not completely.
There was no formal study of English grammar at all until a few hundred years ago, and thus no true rules. The first grammar books and grammar rules were for the most part modeled on Latin. When trying to determine what was "correct" in English, the so-called experts would often defer to what was correct in Latin. In some cases, they deferred to what was
possible in Latin. For example, the rule against splitting infinitives in English came about because it is not possible to split infinitives in Latin. In English, it is not only possible to split infinitives, it is often much better to do so, but many generations of students received failing grades for breaking that rule. The same is true about ending a sentence with a preposition--it can't be don't in Latin, so it shouldn't be done in English, the experts said. That is the rule that supposedly led Winston Churchill to exclaim "This is a situation up with which I shall not put!" English is not Latin, but we still suffer to this day because of misguided people's insistence that it is.
Other rules came about because someone got the bright idea that it should be a rule. When I first got the version of MS Word that had the grammar check, I was frequently flagged on my relative pronoun usage. Mystified, I did some research and found that a little over a hundred years ago someone got the bright idea that the meaning of adjectival phrases would be more clear if we started the restrictive ones with
that and the non-restrictive ones with
which. Some grammar books agreed and made it a rule, but most didn't. Whoever set up MS Word included it. It has never been a part of common usage, though, and I believe MS Word has dropped it from the grammar program.
Our spelling rules are largely a sad result of poor timing. Around 1400, Geoffrey Chaucer's
Canterbury Tales was published in an England that had no grammar or spelling rules. People spelled things the way they sounded to them. Moreover, English dialects differed dramatically from town to town. Few people could read. Chaucer's work was probably the greatest best seller of the day, with scribes working furiously to keep up with the demand for the huge, hand written volumes. Over the next 50 years or so, the English language changed dramatically in pronunciation. It was at that time that the printing press was introduced to England. This allowed for cheap books and made learning to read worthwhile. It eliminated the hundreds of dialects across the land. What was one of their first best sellers? Chaucer's
Canterbury Tales, printed with Chaucer's spelling, then already becoming obsolete. And so for the most part we spell our words today the way they sounded to Chaucer 700 years ago. If Chaucer wanted to write what he heard in a sentence that talked about a "kuhnikt" who carried a "kuneefuh" he spelled it the way he sounded, and today we write
knight and
knife.