Old Ned wrote:
To answer your first question, a 'mixte' is a frame with the twin 'top' tubes running from the head to the rear dropouts. Generally ridden by ladies as an alternative to the 'normal' ladies style frame. I think that for more 'serious' riding this design is a bit more rigid but still allows for stepover. They were built by even the most respected frame builders if required.
It's actually less rigid. A single fat (but thin-walled) tube is a better use of material. That is why bikes have big fat tubes and not pylon-like wire spaceframes. However, the mixte 'twin lat' tubes look graceful. The main force on a bike (beyond rider weight) is the twisting between steerer tube and seat tube caused by pedalling. Tony Oliver's book covers it in great detail.
Ride it or milk it!