a. He was the son of a priest and a historian.
Could one use that sentence if his one of his parents was both a priest and a historian?
Or would that have to be
b. He was the son of a priest and historian.
I think (b) is clear, although it does sound a bit strange to me. I am not sure (a) is not ambiguous.
They both need clarification.
a) would be fine if it went on "...a priest and a historian who met...." or "...a priest and a historian. They met..." As long as it's made clear they met each other, it's clear they were two people.
b) would be clear if you made it, "He was the son of a priest and historian who was ..." - the word "was" being third person singular. Or you could say, "He was the son of priest and historian John Brown."
If he was the son of a priest, and a historian, the comma makes that clear, though we would usually swap the order: He was a historian and the son of a priest.
I fully agree with the points you have given and yes, (a) can be used to say one of his parents was both a priest and historian, but it also might be interpreted as the mother being a historian and the father being a preist or vice versa. However, some may feel that (a) suggests that the son was a historian himself! (b) suggests the same but it is more clear, and removes the ambiguity of the son being a historian himself.
This NGRAM graph shows that, when talking about a single person, both forms are common, though the form in sentence b is currently somewhat more common. Here are two actual examples:
Alec Finlay is an artist and a poet, recently artist in residence at THE BALTIC (Gateshead).
Jenna Boo is an artist and poet who takes you on a magical adventure into her art studio
When talking about a person's parents, I would assume that sentence a would mean that one parent was a priest and the other a historian, and sentence b would mean that one parent was both a priest and a historian.
