OP, your approach should depend on your goals. If you want to preserve your integrity and demonstrate to others (not SDN users, but adcoms) that you might have some semblance of a moral fiber, you should disclose this. If you want to get into medical school at all costs, don't disclose it and roll the dice. More than likely nothing will happen based on your story and the laws people have dug up, but if something does, you're screwed, and you can rest assured that there's very little chance that you'll make it into a US medical school. The offense itself is minor, but lying about it reveals an even deeper character flaw that schools will stay away from. I mean, really, how well do you think the discussion about this will go if your only response was, "well, I hadn't been to court yet when I submitted my AMCAS, and it technically wasn't a state law that I broke."
By the way, I support MJ legalization and think MJ criminalization is a joke. That said, operating a vehicle while high/drunk/other mental status is a bad decision that deserves punishment. You made an unfortunate decision, but saying "I'm sorry and I feel bad" doesn't absolve you of repercussions. It's obvious that you know you're very much in a grey area, otherwise you wouldn't be spending as much time and money researching this as you are.