What is the wordpress Git hub Repo master on beta?

I am not sure what gave you the idea 3.9 is latest, looking at Branches · WordPress/WordPress it hadn’t had commits in 3 months.

  1. Correct for what? If you want trunk then use master branch from GitHub.
  2. Typically you can, but it’s matter of probability and not guaranteed. WP won’t roll back any upgrade changes (such as database schema changes) on downgrade, but they are rarely breaking in such scenario.
  3. Yes, it is automatically updated and receives latest commits in minutes after SVN.