Blue Planet is a trilogy: there are currently two of three parts - Age of Aquarius and War in Heaven.
AoA was initially just a long, however interesting campaign. In Director's Cut, it has been split into 5 acts, but that is rather... symbolical. IMO, the only difference about the acts are several loading screens.
The second part of the trilogy, War in Heaven, currently has two of five acts completed (including voice acting), and the two are bundled together into "War in Heaven 1" campaign file. The third act, campaign "War in Heaven 2", has no voice acting and has been modified pretty extensively in Director's Cut "update". I'd say that it doesn't need pretty much anything aside from voice acting to be perfect (but I am just an another BP player - my view for a perfect campaign might be very different from the developers' one).
So: AoA - completed, WiH - around 2.75 acts of five. And there's a third part of the trilogy to be done
I suggest checking out some discussions on BP board, they are very interesting. And everything you can find written by BP team. I'd say that the game itself is a half, maybe two thirds of Blue Planet.
The project is definitely going on - it's just slow, real life stuff, modding/writing challenges, finishing a book in order to get money for the voice acting, anything else that can be time-demanding.
And 'bout the Durga - just read its tech description. The problem is that in-mission these bombers will have to face many more difficulties. Fighter screen, additional warships, some more sophisticated defences... Not to mention enemy warship stats bumped up a bit - like Delenda Est, devs had to increase Carthage's health a lot in order the mission to make sense. The team will follow the rules of their franchise, not some numbers in game that can be changed with ease in a moment's notice.
And some say that act 4 will be much, much related to bombing. There are even some YT videos on a channel called Dan Bell.