Welliments, I'd stop digging if I were you.
What on earth makes you think "Using a conjunction after a comma" is a problem? OUP house style permits it in lists. Even if you avoid using it in lists of single words it is routinely used by careful writers when elements in a list are longer than one word.
en.oxforddictionaries.com/explore/what-is-the-oxford-comma
The other "breach" of your "rule" is even more ludicrous: it's a pair of commas setting off a modifying clause. Americans have explicit rules about restrictive modifying clauses (which are not set off with commas) and non-restrictive modifying clauses (which are), while we tend to figure it out as we go along. "My sister who is blonde works as a lawyer" implies you have more than one sister and "blonde" tells you which one, while "my sister, who is blonde, works as a lawyer" implies you have one sister that happens to be blonde. So that said: "born of my experience of course" is a non-restrictive modifying clause, and properly set off with commas. That there happens to be an and is irrelevant.
In your world, the following fragment:
My sister, who is blonde, and my father, who is old, went for dinner is (a) wrong and (b) would be made "correct" by removing the second comma. Seriously? Do you believe My sister, who is blonde and my father, who is old, went for dinner is correct? Please enlighten us.
You think people should be spelling humanities as a proper noun? I'm as descriptivist as you like, and think falling back to OED is a weak move. But it's lower case IN THE FUCKING HEADWORD in OED. "2. Frequently in the humanities." and has the helpful note "The humanities are typically distinguished from the social sciences in having a significant historical element, in the use of interpretation of texts and artefacts rather than experimental and quantitative methods, and in having an idiographic rather than nomothetic character." On this occasion, "take it up with OED and every university, everywhere" is a reasonable response.
"Frequent such errors" is perfectly fine. The style police will say "a bit legalistic", but it clarifies that the errors are drawn from the group defined earlier in the paragraph.
I don't have a view on the comma that upsets you. I'd use it. I wouldn't complain if someone didn't. Meh.