I thought the response to the 'late bloomer' conversation was hysterical - I'd love to know how she responded.
I don't know why there is a competition between mothers and the women their sons wind up with. I'm very thankful that mine lives 400 miles away because I can't stand her. If you don't see a move to a different state in your future, you'll have to figure out how to let this stuff slide off your back more easily.
I wonder what would happen if you asked her to explain these little snipes. When she says something randomly rude, ask her "What makes you say that?" It's possible that you are jumping to the wrong conclusion -- and if you aren't, she may get uncomfortable if she is asked to explain exactly why she is being snippy.
I make sure that you are being as polite and kind to her as you expect her to be to you. Do you snipe at her too?
If you aren't doing this already, let her have some alone time with her son. I started doing this with my boyfriend's mother and it's worked out wonderfully. She is happy to see her son one-on-one and I don't have to deal with her as much