Grammarians descend. You could see this coming. One writes:
Your reader's example - "The $1 million was divided between Mary, John and Frank," is a lot different than "The $1 million was split between Mary, John, and Frank." - is completely nonsensical.
It suggests that, without the serial comma, it is necessary to read everything after the first (only) comma as a single noun phrase; so, "The $1 million was divided between [NP1], [NP2]." But substitute any single noun phrase into that, and you can see it's ridiculous: "The $1 million was divided between Mary, John" is grammatically incorrect, and so can't be a viable way to read the sentence. Whether or not the serial comma is there, the money is being divided three ways.
Weintraub's example, as well as your reader's "Ayn Rand and God" example, points to a very specific occurrence of a serial list: where the first noun phrase in the list ("his two ex-wives," "my parents") suggests a specific count of individuals equal to the number of noun phrases following in the list. If the apocryphal dedication had been "To God, my parents and Ayn Rand" or any order other than having "my parents" first, there would be no confusion regardless of the presence or absence of the serial comma.
Not so fast. Another writes:
The dedication quoted by your reader - "To my parents, Ayn Rand and God" - is amusing, but there is a similar line that is ambiguous with the Oxford comma. The variation, "To my mother, Ayn Rand, and God," has multiple meanings with the comma and is unambiguous without it.
There's another problem with your reader's grammar lesson - word choice. "Between" refers to a relation of two, where "among" refers to three or more. So, "The $1 million was divided between ...." already implies a sense of two and not three - though I think common usage would indicate in both examples that the three would each get a slice of the pie. But more importanty, there's nothing to formalize equality here, and Mary could get $1 where John gets $999,000 and Frank $999 (sorry Mary!).
One of your readers wrote:
When the quoted text DOES have punctuation and it comes at the end of the sentence, you omit the final punctuation:
The small child asked, "Where's my mommy?"
The small child asked, "Where's my mommy?".
I think the second one seems more correct. The question mark is part of the quote, the period ends my sentence.
Maybe. But here's another example consistent with this logic:
Why did the small child ask, "Where's my mommy?"?
Sometimes punctuation rules are designed to avoid unintended consequences.
(Vampire Weekend differs with Weintraub.)