I have a Snark also and have also been out with 2 kids in it, but thery are smaller than yours. Are you sure your "newly made daggerboard" is the right size and is rigid enough?
When you point too high, you will stop moving forward and wind and current will move you. It is generally not a subtle change; you should notice.
I do not encourage anyone to ignore sasfety ratings, but I do think the Snark's is rather conservative. I had my two boys and a fair amount of gear with me in it in light winds on a local lake and I crusied out and around and back to the car on a day where the skiff at the Sunsfish rental place was making lots of trips to tow people back up wind.