Your analysis is faulty. Even if projections of 50% underage users are correct (and it's unlikely for a number of reasons), you're overlooking the fact that in the case of legalization most of those would be getting their pot from "diverted" legal sources, the same way that underage drinkers today get their booze from Dad's liquor cabinet, or with false IDs or from an "under-the-table" liquor store clerk, or via friends who are of legal age.
What's the last time you saw an underage drinker get bootleg liquor from someone's illegal still? No, they get Budweiser and Jack Daniels from legal producers/distributors -- they just find ways to get around the measures put into place to keep booze out of the hands of minors. That's a lot easier than finding and maintaining a complete black market "alcohol cartel", *and* it's safer, the supply is more reliable, the product is more consistent, and of higher quality. Who in their right might would buy "back alley booze" of unknown safety and quality, when they could get Jose Cuervo Tequila from a friend over 21 who can buy all you want?
The same factors would be at work in the case of marijuana legalization, no matter how large the "youth market".
AND, they'd be much harder to catch, in that it would be perfectly legal for the dealers to own, carry, or grow pot -- you'd have to catch them actually selling to kids.
Not at all. Black market pot would be easy to distinguish from legally produced pot, in the same way that it's not hard to tell bootleg liquor from a case of Absolut Vodka.
Currently, underage drinkers represent only 10% of all drinkers. Imagine if that were 50%. Do you think that market could be satisfied by "booze from Dad's liquor cabinet, with false IDs, from an "under-the-table" liquor store clerk, or via friends who are of legal age"?
I don't.
"Black market pot would be easy to distinguish from legally produced pot"
Not if adults are allowed to grow their own. And it's very easy to grow very good pot -- you can't say the same for beer, wine, scotch, bourbon, etc.