Ok 1, it's already illegal for anyone of any age to smoke. It's illegal unless you have your 215. If an army vet comes back at age 20 and gets his script he can smoke legally under 215.
Smoking in public also depends on the county that you are in. 19 doesn't take away the right to smoke in public that 215 gives you, and in counties where it is already legal to smoke in public (SF) that isn't taken away either.
2, The prop does not ommit the cultivation section. In both section 7 (where it only says sell and posses) and 8 it ALSO says except by laws permited by 11362.5 and 11362.7-11362.9 which give you your cultivation rights.
3, It is very unlikely that nation wide corporations will sell weed. The feds would be all over that and shut them down. Remember that this isn't going to be federally legal, and our nation's government needs money so it will jump on any chance to screw over a nation wide business with a huge law suit.
I will admit that the prop is confusing, but if you spend time reading it and analyzing it, you will realize that it is not screwing anyone over except for people who rely on selling bud for a living.
Also, cities are already starting to tax medical mj. SF and Oakland have started it, and Berkeley and Santa Cruz are soon to follow. Once the medical triangle starts taxing other cities will follow. There might be a tax, but take something like peanuts. They costs more money to make than marijuana and they are dirt cheap. There's a tax on them, but it's not significant at all. Once bud starts getting mass produced LEGALLY (it's already mass produced illegally in the emerald triangle) then the prices will plummet and the taxes will not raise the price over what it is now. Personally, I have to pay $50 an eighth for medical quality buds. After 19 passes it will make huge grow factories legal (there is already one in Oakland) and it will make buds dirt cheap with a tax. I don't really care if I pay $25 an eighth plus $5 dollars tax, I still save $30 (This is a hypothetical situation).
The prop also won't make buying from your street dealer any more illegal. If that's how you get your bud then you're in luck cause you will be unaffected. You will just be paying more for the same product you can get in the store.