1) I don't think there's much to discuss about it. For good or bad teenagers
will drink whether they can buy it or not, whether it's legal or not ... and if you're lucky they'll start smoking weed or what worse because it's easier to get hold of
2) Parents: yes. I don't know how it is other places, but in Norway, if they can't buy regular alcohol, they end up buying home made 96%, sometimes even mixed with some methanol. And if you have to ask your mom for beer maybe you would ask for a bit less, bring a bit less and drink a bit less than if you had gotten it yourself
3) No, this goes with #2. If they can't buy it in the store, but
can ask their parents, then they can legally get proper alcohol. Now of course I think that the 21-year limit in the US is completely ridicilous. As my American friend's dad said to the 18-year old Marine who came to their place: "If you're old enough to die for your country you're old enough to drink." One might of course argue that some limits are also too low (Denmark: 15 for beer). I think 18 is good.