I was arrested for transportation of alcohol by a minor (I am 20), but was the arrest legal?

I (20) was driving a car with 4 other individuals (each 21 or 22), when we were pulled over at 11:30am on the highway. Walking up to the car, the officer saw three 6packs of beer in the back of the car (it is a hatchback, so the alcohol was in the equivalent of the trunk). After asking for license and registration and then asking my age, the officer asked for the ages of everyone in the car (again all over 21). All of us are from out of state and had no idea the NH law forbade me from driving with the alcohol in the car. We assumed that since they were 21, it would be clear the alcohol was theirs (also, I had absolutely nothing to drink, while others in the vehicle had, so I opted to drive as DD). I was subsequently arrested, the alcohol seized, and the car impounded. Are there any grounds for dismissal? Additionally, at no point during the arrest did the officer tell me why we were pulled over in the first place and no ticket or fine represented a reason for being pulled over (Grounds for dismissal on the basis of no probable cause?). I was not read my rights. I was in handcuffs before I was told what I was under arrest for. The other passengers in the car were detained in a police vehicle (although not put in handcuffs) and refused access to a restroom (false imprisonment?). The one additional detail is that a backseat passenger was charged with an open container violation (although it was clearly out of my reach, and my arrest and fines do not reflect the open container violation). Is there any opportunity for me to have the case dismissed and if not are there diversion programs available that can keep the arrest off of my record and hidden from background checks?