It is my impression that when drinking age restrictions are introduced, drinking among the (now) underage population goes down. Not disappear, but goes down.

My personal experience is also that 17 year olds in countries where the legal drinking age is 16 drink more than 17 year olds where the drinking age is 18, but I don't have numbers on it.

It doesn't need to be 100% effective in order to work.

Yeah I'd say it works. I've seen many minors doomscrolling on their phones. I haven't seen any with a beer in hand. That's not to say there aren't any, but there are fewer.

Do minors drink less than they would if we allowed (or even encouraged) them to drink or use drugs?

Dutch teenagers are less likely to smoke, drink or have sex compared to teenagers in other countries. Despite the nations relaxed stance on drugs and sex work. So making something legal could make it lame for teenagers.

Not sure, but they would do it more safely, thats the argument for safe supply :)