It really depends on your own personal moral system (assuming ethical relativism). In order to answer your question, I would need to know what you consider moral. I'll attempt to infer your morals from your post, and then I'll try to answer your question accordingly.

It sounds from your post like you're torn between two alternatives, both of which you consider moral, but which are mutually exclusive. On one hand, it seems that you're morally devoted to the causes of atheism and truth-seeking; thus, you desire to convert others to this cause. But on the other hand, you're morally devoted to your friends' happiness, and you realize that if they do become atheists, then they will lose their social grounding (not to mention the emotional benefits they receive from being religious).

It sounds like you're very devoted to truth-seeking, and that you believe atheism to be the truth. (Side-note: as a Bayesian, I distrust anyone who claims to know "the truth". The point of Bayesianism is that we don't know the truth; all we have are probabilities, and thus we can approach the truth but never attain it.) Anyway, given your devotion to truth-seeking, I would expect you to want to avoid Dark Arts-ish methods. If atheism is true, then we (and your Catholic friends) should want to believe that atheism is true, but we should want to believe it because of empirical evidence and rational argument, not based on the words of some authority figure (especially since authority figures have proven unreliable in the realm of religion).

If you deconvert your friends using Dark Arts-ish methods, but you don't teach them the virtues of truth-seeking, then atheism will become just another religion to them, handed down by new authority figures (you and "Science", for instance). They'll accept atheism in the same way they accepted religion: with blind faith. If your goal is truth-seeking, then you should want to teach your friends skepticism, not atheism. And if you're so interested in converting your friends to atheism that you would sacrifice the virtues of truth-seeking, perhaps you should re-examine your motivations.

You note that the God issue is a source of tension between you and your friends; thus, I suspect that you want your friends to be athiests because it would relieve social tensions, not because you are devoted to spreading the virtues of truth-seeking. Because you are considering using the Dark Arts, it seems to me that your appeal to truth-seeking is a rationalization. So what you're really asking is, "Is it moral to make my friends' lives very difficult in order to relieve a social tension that I find unpleasant?" Most moral systems would say "no".

Perhaps I'm being a bit unfair to you. Perhaps your true goal is to encourage truth seeking, but it's easier to convert atheists to truth-seekers than it is Catholics. Or perhaps you believe that atheism will make the world a better place by eliminating holy wars and other problems caused by religion. If that's the case, then I apologize for the harshness of this analysis. Also, fwiw, my personal moral system says that converting your friends to atheism would be wrong, so I'm likely to be biased. Take this (and everything else in life, of course) with a grain of salt, and good luck to you with whatever you decide to do. =)