I agree that the database would be expensive to implement. On the other hand, taxing marijuana and all the money that could be saved from not fighting marijuana dealing anymore could perhaps finance the database system. I am not a security professional, so I don't know how to implement a system that could authenticate users without identifying them, but that seems to present lots of interesting technical problems.
I agree that if there's no alcohol database, then there should be no database for weed either. On the other hand, alcohol is tolerated by society much more than weed. This makes no sense, but it's a reality. I would say that being pragmatic would be a good idea: introducing a database would tame the "fears" of those who claim that legalization would "corrupt" the teeanagers and our society's "values". Sometimes it's better to take little steps. Legalize but control consumption using a database. A few years later, people will have gotten used to the new paradigm that there might be no objections to abolishing the database. I say, legalize one step at a time because it increases the chances of success.
As a total aside, this came up in an episode of Weeds - our hero Nancy Botwin managed to discourage her clients from purchasing medicinal marijuana just by suggesting that there was such a central db.
This is wild speculation, of course. I just wonder how the government will be able to enforce certain laws when citizens are able to synthesize chemicals on their own homes, for their own consumption.
To address 2) there's a lot of problems, it would be expensive to implement, there's a lot of security issues involved, it seems unnecessary - alcohol and cigarettes cause many more problems; they should get this treatment before weed does. On the other hand, I feel like the system you proposed may already exist in rural small town pharmacies for antihistamines which can be used to produce crystal meth.