That's a pretty tricky situation~ not sure the kids would like it, but perhaps "wristband" the 21 year olds like they do at bars, etc. and if any adults see a non-wristbanded person drinking, to say something. And have someone bartending giving alcohol only to those of legal age (indicated by the wristband).
What a tragic situation it is about your DD's boyfriend~
I don't know what the laws are where you live, but here we have hosting laws~ the hosts are accountable for any underage drinking, whether the hosts are aware of it or not. Too much of a liability for me personally to take the risk, so I wish you best of luck with your situation.
Congratulations to your graduates~