I"m not sure of the right answer, but I thought i'd commiserate! My mom is much the same way. She mentions the stomach sleeping and all the other horrible things that they used to do with children 30 years ago and refuses to accept that times have changed and things have changed quite a bit. My mom came home from the hospital with me on her lap in the car for goodness sakes! Anyway, even while I was trying to get pregnant and through our infertility and miscarriages, my mom said insensitive things like "when you're a mother, you'll understand" and she gave my sister a mother's day gift right in front of me at my house right after I had a miscarriage. Couldn't she at least be discreet??? Anyway, now that the babies are here, I get the same things that you do and my mom also badmouths me to her friends and family. One thing though is that most people know not to take her seriously. You might be surprised at what they think of the things that your mom says. Since my mom's attitude does not end with me, everyone pretty much knows that she's full of it

. As far as the stomach sleeping, this is what quieted my mom:
After my twins were born, they spent two weeks in the NICU. While in the NICU, the nurses put them on their bellies some of the time to sleep, they said it helps food digest better and the babies actually sleep more deeply on their stomachs. So deeply, in fact, that the reason that SIDS is attributed to stomach sleeping is because some babies sleep so deeply that they forget to breathe. The let them sleep like this in the NICU only because they were hooked up to all kinds of monitors that would tell them in a split second if they weren't breathing, etc and they told us to NEVER do it at home! So, when my mom argued with me about the stomach sleeping, I told her that she's right, that babies like it better, but because they do forget to breathe because they're so comfortable, that's why we're not supposed to do it. That shut her up and now she tells anyone that's expecting why they should sleep on their backs.
Another funny thing. My mom works with a woman that has two younger children, so every time I do something, my mom compares me to this lady and says "so and so said she did that at first too, but then she realized it was total crap and went back to the other way". I feel like she's always using this woman to prove her point and I wonder if she's just making up stories about her just to try and prove her points. My mom argued with me about buying a changing table, using nursery water, waiting to start solids, putting cereal in the bottles, etc. It never ends. I'm not looking forward to the coming years! My babies are only 6 months and i'm sure I have a lot of unwanted advice ahead of me!