Muirenn, I'm a hair shorter than you (5'6-3/4"), and I have the same problem you do. I need a pretty short top tube, and even some WSD bikes are too long.
My Giant is a 50cm/M, but they run big, by the looks of it--and the Avail's geometry isn't terribly different from the Defy.

My knees weren't terribly happy with the test rides, but that may well simply be because the seatpost was adjusted to "eh, close enough" and I have crappy knee ligaments. If it were properly adjusted (ie, when I'm actually serious about buying), it might well be a different story. It felt good otherwise (even better than the Treks), so I'm still keeping it on my "list"

I did in fact try out a Quest Femme (lovely bike), but I'd need a 54, and that's something that the shop doesn't stock and wouldn't until I'm serious about buying. I had the same problem with trying out a Cannondale.