AJ:
You wrote: “Unfortunately, too many families have let mental issues go untreated and unacknowledged because they were worried it would ruin the shidduch opportunities of their other children”
You are certainly correct about that. What these families forget is that a psychiatric illness can be better covered up and kept secret if it is treated and there is absence of symptoms. Otherwise, one can safely say that there is no history of psychiatric illness, but the candidate is nuts. That actually reduces chance for a successful shidduch.
Having noted this, withholding such information to facilitate a shidduch is, in my opinion, criminal behavior. I believe the Chofetz Chaim spoke about this, and that withholding such information is wrong. I leave it for others to quote the citation from the CC.