I too have very short legs (mine are even shorter than yours, I think). I'd also like a mixte, but have been really lusting after a diamond frame. A 48cm one fits me ok and they do come up now and again. In fact, I have a Bianchi on the way which will be 48 or 50 cms and I hope it will work for me. Honestly, I think you just have to keep looking obsessively until what you want comes up at the right price (it did for me, but I had to pull a favour to get it as this bike is 8 hrs drive away).
Terry bicycles were also suggested to me, though they are out of my prce range (even the older ones). But worth checking out. Some of the frames are truly tiny.
There's a 48cm road bike in a thrift store near me at the moment for $130. I didn't pay too much attention to it as it wasn't as compelling as the Bianchi, but I think it was of ok quality.