I think there are several valid interpretations of "A, B, C or D":
I don't understand that argument at all. The list construction is separate from the choice of "and" or "or". That choice tells you how you are combining the list elements. The list construction is there to make clear what the elements are. If you want three choices, such as:
* red
* yellow
* green or blue
then you need "red, yellow, or green or blue".
A more common example is where the list is with "and". Here, an imaginary rule seems to be emerging that you can only have one "and" (before the last element in the list), but that's not true at all; you need an additional "and" if one of the list elements involves more than one item/person. Imagine for example that Uncle Henry is giving gifts to three nephews and nieces, but John, who is married, gets a gift shared with his wife Sally. So the gifts are for "Mary, Peter, and John and Sally". That's the same logical approach as for the colour list above. Indeed, if Uncle were deciding whether to give a single gift to "Mary, Peter, or John and Sally", it would be
exactly the same construction.
There are Oxford commas in my examples because those are used where they make scanning the list easier; "Mary, Peter and John and Sally" means exactly the same, but is just plain harder to read. Even without the Oxford comma, it cannot mean that Peter, John and Sally are themselves all receiving one joint gift, because then you'd need another "and" before them. At that point, the whole thing would become so complex and ambiguous to parse that you'd have to do something different - assuming, of course, that your aim was to convey meaning clearly.
