They haven't given a specific date for the beta, but I imagine it would have to be pretty soon. However, that does not mean you should not start playing Sins, general experience managing your economy and fleets will still be relevent no matter what the Beta does. If you're intimidated by the military aspect, you can play a map with no AIs (or just an ally on locked teams) and practice playing the game that way. For my first game I played with no AIs but left the pirate raids on to get some practice with military and defense matters.
As for features list, they said that Diplomacy would add more game modes, a new "diplomatic cruiser" (though we have no idea what it does), a new diplomatic tech tree, the ability to give the AI "missions" like the AI will give you on unlocked teams, and presumably a major additions to how you can interact with factions that are not full enemies. Overall still pretty vague, but Entrenchment changed a lot from Beta to final release, so things will probably be added, changed, or even removed as necissary.
I think some military will still be needed for initial expansion and defense, but I wouldn't be surprised if you will be able to win without destroying the main enemy fleet and/or taking over most of the map, at least for certain game modes. But all of that is just my speculation.
