I don't see the allure of a mixte frame, unless of course she's going to be riding it to work in a skirt and all. And anyway, most mixtes aren't small enough.
Small frame sizes like 46 cm may reduce the distance between pedal and seat, but will not necessarily give a better fit since the BB and headset are typically no lower than on a full sized bike. So even if you find a mixte with a small frame size, there's probably no benefit, since you will still end up with a bike that doesn't fit.
Terry gets around this problem by reducing the front wheel size, which is fine, but why not a smaller rear wheel as well? It not only looks goofy, but means you have to stock two different tire and tube sizes. The fact that they do it anyway tells me Terry's "woman-specific frame geometry" is basically a marketing ploy, a flashing light that says "serious bike for a seriously small person" in order to justify the price.
You are rating "form over function" even understanding the limitation. It is a LOT deeper then you make it out to be and most of it revolves around available components.
This is very true for the racing bikes maybe less so around a leisure rider who is not trying to be competitive. In the perfect world you would just wave the shrink ray and everything about the bike would be smaller...doesn't work in the real world.
The common available components are the problem.
First The crank arm length determines BB height as you said. The lowest you normally see a BB is 265mm (10.5 inches) for the 165mm short "common" crank size. You will be VERY hard pressed to find quality cranks with a shorter arm at a reasonable price. This already starts everything off with a disadvantage when a person under 5ft tall may need something in the 155mm range for their leg length allowing for a BB high to be around 250mm (9.75in). So they are already screwed on that figure. You cant get the BB closer to the ground even with a smaller tire.
Now lets get into gearing. Everything revolves around the rear wheel size. 27-700c being the most common size in a competitive bicycle. That right there means for a short person to be competitive they are forced to those sizes or they are at a major gearing disadvantage.
The Diamond frame is the best rigidity verses weight design but its smallest size is is determined by the wheel size.
So now you are left with how to get the bike smaller but fit the 700c and 165mm cranks. As you down size the frame the top tube gets closer to the down tube, The closer they are the more flex you are going to have in the frame at the head tube and make it spongy as you get below 54cm the angles really start to suffer and you have to angle the top tub but because the top tube is short it doesn't give the stand over clearance advantage under 50cm.
On a bike with a 700 front wheel you start having toe over lap problems as the front wheel moves closer to the BB as you adjust reach for the shorter torso length. Some of this they solve by screwing with seat and head tube angles and then really short stems to compensate and they all put the rider at a less then optimum riding position.
Now understanding all those limitations when you throw a 24 on the front and shorten the seat tube while retaining 700 rear wheel you get the gearing but have fixed the toe overlap problem while retain the better geometry that bikes over 56cm have. Its "Function following form".