The art I've seen the most women involved in has to be aikido. Alot of it, I think, has to do with the "gentler" focus of this art. It allows women to step on the mat without having to confront as much of the macho bs that goes on in the arts. The problem is that the "gentler" and "more ethical than thou other artists" extremists in some of the aikido dojos perpetuate a false confidence and misconception of what will happen on the streets... but the same could be said of other arts.
Having make that criticism, I think aikido perhaps succeeds in some ways with women more than other martial arts. It provides a gradual entry into martial arts and self defense mindset that is not only accepted but often actively endorsed. I have seen women beginners who were terrified of physical contact, especially with men, become more at ease with it over time. There is no doubt in my mind many of these women would not survived the initial months in some of the hardcore arts/dojos. Some of the women aikidoka become become very "hardcore" within the parameters set by the dojo. And they can find success in a martial way, as you pointed out, which favors the lower center of gravity most women have and ability to blend with the power of the opponent (musubi). Now, if only more aikikais would just introduce/allow for more freestyle -- not stylized "randori" -- and resistance in the uppper level training, female students as well as the male students concerned with self defense could be much better prepared.
Of course, ultimately the individual skills and mindset kick in and can transcend any art. There were two female shidoin (assistant instructors) I highly respected for their martial skills on the mat. They were not aversed to a skillful use of the atemi to distract/offbalance an opponent for a hard throw. How did they learn to atemi since striking was rarely practiced? Don't know for one, but the other had a brown belt in a hard japanese style before taking on aikido. The latter in fact dropped a male attacker on the street with an atemi and beat the crap out of him (as witnessed by a dojomate and friend.) Well, so much for the gentler approach...
>>The question still applies then: What would a woman do in a "real" situation when karate fails to do all that she expected?<<
This is an issue of the mindset and not the art. It applies to all us, male and female. Forget the specific techniques and moves. Keep one mantra in the mind: I will fight and fight until I or the opponent can fight no more!