On the last point, it's a lot harder to manage than you're making it sound. A public org isn't going to gate signups.
Like email, I sign up for a Mastodon account on a server that suits my tastes. I then follow other accounts, on other servers, using a client - I use Ivory on the mac & iPhone (because it's made by the same company who made Tweetbot) - and those accounts show up in my feed in chronological order, like Twitter in the old days.