Is Mannimarco King of Worms in the game? Are there even Liches in the game at all, or plans to include them if they don't exist?
I haven't seen Mannimarco, so he probably is not in the game (al least not at the moment), but his nemesis Galerion is (he is the head of the mages guild at the start date, but tends to die a few years in). There definitly ARE liches in the game (represented in the form of a trait), although I can only remember seeing this trait on the initial ruler of the Frozen Horde. There is a possibility for Mages to become necromancers though, and I'd be surprised if the devs wouldn't eventually implement a way for such a character to become a lich, if they haven't already.
Or the Psijic Order, and perhaps mechanics to inluence the Mages' Guild politics?
No to both. The Psjiic Order, along with the island of Artaeum, vanished before the games start and only returns later. Sadly, it is not possibly to model this with the game engine, so I doubt we will ever see Artaeum or the Psijics (as they are very isolationsist and wouldn't really add to the gameplay).
The Mage's Guild is included in the game, but only plays a very minor role and doesn't really have any politics*. It basically acts the same as the Varangian Guard for Norse in the vanilla game: your children can ask you to become members of it and may or may not return.
What about Numidium?
No, and I doubt it will ever be included, because it would be extremly hard to manage this within the limits of the Clausewitz engine. BUT there is (or was) the "Anumidium Worship" religion, which was only "obtainable" through the use of the console.
*I don't now about the exact requirements to join the Mage's Guild, but I think it should not be possible to join for characters with the necromancer trait. So I guess this could count as "politics", but you can't change this.