I don't see the first question as particularly relevant. Suppose the prediction is that after deconverting they will be grateful, what would that prove? (Cf. the Gandhi murder pill example: Killer!Gandhi is happy that he took the pill.)

The second question is a better one, and I like the distinction you make for your case. Generalizing, I'd say it is ok for thelittledoctor to give rational arguments against their friends' beliefs (as long as he or she does not do it in a pushy, obnoxious way, but when the topic arises naturally) but not to use "Dark Artish-levers".