Maybe you should talk to Dan Callahan, he'll be happy to answer your questions in more details I'm sure. I'm saying that because you sound like Persona could benefit you and Persona certainly could use more people like you criticizing it :)

So this comes back to tying accounts to emails: Well, this is the user's choice. They put their internet life into the hands of Google or Yahoo or whatever by choosing such an email provider that may go down at any time. It's completely reasonable and does not actually put YOU in a position where your sites locks users into a third party, you are just giving more choice.

As for activity: although it's not on git, there has been renewed interest in Persona lately. And until a better alternative comes by (which won't be for a long, long time) I'll defend persona tooth and nail because, it may not be perfect, but it is far better than what is currently taking over the entire web.