I stand by my own suggestion. It's an aesthetic preference: infinitives with objects as subject isn't my cup of tea, and I would say the sense of the original English sentence is conveyed; there can be no doubt that the act of defending is what triggers the strengthening of the soul even though it is not syntactically subject. By making it an instrumental, we also avoid the weirdness of an action (that it, something that is very abstract from a physical point of view) being the semantic agent: it is only a facilitating concomitant quality. As you yourself write, it is a strengthening that takes place inside of you (the soul grows stronger) as a necessary evolution due to the circumstances, not the actual action that moves inward; it swings both ways, but the quality of protecting stems from the very soul that strengthens.
This is a babble. My English is not at a level where I can thoroughly explain my complex feelings about syntax and souls

Let this be the bottom line: it's basically a matter of taste; as I have stated in another thread, translation is the act of transferring a set of semes (basic units of meaning) from one tongue to another, and the way in which Adrianus translates your sentence is every bit as sound as my suggestion. His
impotentes might even be a better word, although it may convey a connotation of someone who is unable to control his or her passions, but then again, almost any word will have a negative definition next to the morally sound one.
I'm sorry to hear about your and your wife's struggle. There is nothing to be done from the other side of the globe to ease your situation, but I can wish you everything propitious and the reassurance that your family is now a global concern

EDIT: I choose the masculine
animus, Adrianus, mostly because it seems to be the preferred one in L&S at first glance, and then of course because of one of my usual vague feelings that it is right. I will not defend my choice of gender, since both can be used, even interchangeably.