I don't see how being able to get something out of mindkilled people is the first step in making them less mindkilled. You got what you wanted and paid for it by reinforcing their beliefs -- why would they become more likely to change them?

You're not substantially reinforcing their beliefs. Beliefs entangled with your identity don't follow Bayesian rules: directly showing anything less than overpoweringly strong evidence against them (and even that isn't a sure thing) tends to reinforce them by provoking rationalization, while accepting them is noise. If you don't like Christianity, you wouldn't want to use the Christian argument for charity with a weak or undecided Christian; but they aren't going to be mindkilled in this regard, so it wouldn't make a good argument anyway.

On the other hand, sneaking new ideas into someone's internal memetic ecosystem tends to put stress on any totalizing identities they've adopted. For example, you might have to invoke God's commandment to love thy neighbor as thyself to get a fundamentalist Christian to buy EA in the first place; but now they have an interest in EA, which could (e.g.) lead them to EA forums sharing secular humanist assumptions. Before, they'd have dismissed this as (e.g.) some kind of pathetic atheist attempt at constructing a morality in the absence of God. But now they have a shared assumption, a point of commonality. That'll lead to cognitive dissonance, but only in the long run -- timescales you can't work on unless you're very good friends with this person.

That cognitive dissonance won't always resolve against Christianity, but sometimes it will. And when it doesn't, you'll usually still have left them with a more nuanced and less stereotypical Christianity.