The space marine model hasn't been rigged or animated yet, so unless you want to make a diorama of some very bored looking un-armed marines you'll have to wait.
In "Synapse" as it is currently planned all the AIs are on the Tyranid side, in order to give the effect of the Tyranids being this huge swarm and the Marines being a comparitively small, elite force. AIs on both sides might make it feel like a bigger battle, but it would loose the aspect of fighting the swarm.
A control points system like the one you suggest hasnt been included in the planned modes because it feels like too artifical for the 40k universe, like game mechanics for its own sake.
The devourer/flesh borer/spine fist variations dont sound like they would offer significantly varied gameplay to be worth doing them all. I'd be inclined to go for the most average one and put the other two on the back burner.
That being said if we implimented 5 different classes of gaunts we might not need to make any other tyranid player models (for some of the modes), which would be a huge effort saving...