I'll grant you that is a clever point. However, it would equally valid to require someone to prove the preacher wrong. Since neither is in fact provable, I doubt any serious court would entertain either argument.
A court would entertain arguments about whether the deceased was a drunkard and fornicator. I don't believe anyone has disputed these claims. If you can establish that the deceased was not one of these then you would have grounds for libel and I will withdraw my objection.
Besides, I haven't said he should be sued for liable, although if he commits it he should. I have said that he should be sued for fraud and infliction of emotional pain. Possibly breach of trust or some such.
If someone's kid has nightmares about this for years, he should pay damages.
A funeral is a good place to look for people who are dangerously close to suicide, it is no trivial matter.