For all questions security-related, check our Security Roadmap: https://github.com/pagekite/Mailpile/wiki/Security-roadmap

The DIME story is largely "wait and see", but we've reached out to Ladar and the stuff he talked about at DEFCON sounded very promising. We'd love to be one of the first apps to support it.

So have 100% of the beta goals been met?

Yes, everything under Beta on the roadmap "should work" in the release. Of course, there may well be bugs, but that's a pretty accurate description of where we think we are right now.