As far as should of, I think you're both right.
Except in his theory, the mind would have to make the leap to change 've into of. Hard to make that leap when it's right in front of you on the page.
Not to make a big deal of a small point but this is going to necessarily be a picky thread. The most spirited, passionate, screaming near-violent arguments I've ever been involved in have been in newspaper editorial meetings.
I believe he meant the same thing you did but did not express it as concisely as you did. It has to do with the contraction and how it sounds when we express it verbally. Should've sounds like should of. So you are both right. You both meant the same thing.
I see what cher sayin'.