Aries –
You are 100% right about not disclosing info to a stranger. I’m in a similar situation, where I know (suspect?) negative information about someone, and when called about that person, and asked about personal / medical matters, I tell them, “I really don’t know the answers to these qustions, call Rabbi ______, who knows the family very well.”
The rov knows what and when to disclose. For all I know, he might withhild the info until after date # 1, and then call the other side back and tell more. If its a dead end, why spread the private news?
Still, this is a tough call. If it were my child, I’d ask to meet the family face to face, before the engagment is announced and ask them, “is there anything you want to tell us before we finalize things?”
That way, if I find out later, I can always know, I tried. In the end, its in Hashem’s hands, so try as you might, you still need to rely on Hashem.