I dunno Eb, I think you are the product/victim of your own biases and perhaps some mythology told to you during your own driving development. I don't think there is anything at all wrong with driving barefoot. Unless your foot is the tender equivalent of the Princess from the Pea fable, or you have lined your pedals with thorn branches, then I doubt there is the slightest risk. People that have to slam their brakes are doing so instinctively, and I don' t think they run through a "Oh my I should brake but don't wish to hurt my foot" algorhythm, but rather a "ZOMFG I HAVE TO STOP NOW SLAM SLAM SLAM" process.
Further, I suspect that driving barefoot is safer than driving with high heels, though I admit my experience in the latter is limited.
I see you have a sense of humor about it, which is good because I just can't validate your view. I think you should pick more important battles. Does she signal? Does she adjust the mirrors and radio BEFORE she drives? Does she check the blind spot before lane change? Does she talk on the phone while driving? There are hundreds of avenues for you to exercise your mandatory parental nagging quota that will result in actual improved safety, your barefoot issue just isnt one of them.