Longer top tube doesn't mean more stretched out. It just means longer. Unless I've missed it, this is the first time you've mentioned her size which, I agree, can limit your choices. You'll need to actually look at geometry, the drawings, to really get an idea of headtube length and necessary rise of the stem to get the saddle/bar drop.
There's lots of bikes out there with 51cm top tubes but maybe not low end AL and carbon as a frame-only. Which begs the question why that material?
It's sounding like she's new to cycling? You want her to be comfortable, have fun and fall in love with cycling? Sorry, I'm just reading between the lines here.
Have you considered something like a Gunnar Sport?
http://www.gunnarbikes.com/sport.php
That would be a fabulous first bike with a Campy build. At least one of our TE members has a Roadie and I've heard her (and others) comment on the comfort over and over.