Being young as I am, and still in studies, I will make no claim to mastery of Catholic moral theology. However, it is obvious to me that the principle of double effect cannot apply in this case, and that your analogy to the case of ectopic pregnancy is incorrect.
In the case of ectopic pregnancy, an abortion is not performed. Instead, the fallopian tube (which, as I understand, typically becomes infected in an ectopic pregnancy) is removed, and the fetus (which may already be dead) along with it. The death of the fetus is the secondary evil effect of the moral object, which is the prevention of the death of the mother through the removal of an infected fallopian tube.
In the case discussed in this thread, the primary action is the direct commission of a moral evil: the use of a contraceptive. St. Thomas, and all other theologians who argue for the soundness of the principle of double effect, make abundantly clear that an evil action may never be performed for the sake of a good effect. It, frankly, seems that you either misunderstand the principle of double effect, or you are intentionally misusing it.
I suppose I'll say one other thing: the word martyrdom came out of your e-mouth in a post quite a ways up on this thread... so how about it? Is it morally licit for a man or a woman, under the threat of death for being a Christian, to deny the faith in order not to be killed? By your reasoning, that which is "objectively" a mortal sin may be done in order to save a life. Either you believe that the use of a condom is not really a mortal sin, or you must, by your reasoning, accept that public denial of the faith to save one's life is morally acceptable.
But, as one other poster pointed out, this is a matter for the woman and her confessor, which is what I should have said in the first place. Much can be worked out in the internal forum.