I'll try and throw my hat into the What The Hell Happened ring.
My own interpretation was that choosing Sebastian as it's host for the Cheetah and rescinding it's power from Minerva would be an attempt on the part of the plant-god to bait Minerva into re-establishing a system of tribute.
That comes against the point about self-awarness, so here's how I figure it: Urzkartaga doesn't have sentience in the sense that it doesn't have a brain, it doesn't have language, it doesn't *think*. It's a sort of entity that exists as an intangible force of emotion and instinct, and reacts to external stimuli accordingly.
The death of Chuma left the ritual without a guiding priest, and then Circe cut Barbara's dependence on blood rituals altogether. Cut off from communication with it's mate, seemingly unable to actually DO anything in the physical world of it's own volition (other than wilt a bit), and threatened with irrelevancy, it might sense a danger of being forgotten. It used to be a revered object of worship after all.
If it can sense that much, then maybe it would withdraw it's gift from Minerva and bestow it to someone unfitting as a desperate attempt to seize her attention - or maybe more appropriately, an attempt to assert it's dominance over her. Take away something someone wants, and it'll come chasing.
As for Circe and Psycho, the best explanation I can come up with is that Circe can't be *partners* with anyone, love or no love. So shacking up with Sebastian would be great, but she would want to keep her thumb on him.
I imagine that he'd approach her for companionship so she accepted, but needed keep an invisible leash on him. Say she learned of his thirst for some sort of power, then recalled Minerva and the plant god. Perhaps she made a backroom deal with Psycho - she would establish a line of connection between Urzkartaga and Sebastian to lull him into a sense of accomplishment, and then Psycho would take over in Sebastian's head as the 'voice' of the deity to fully convince him that this was the real deal. Perhaps Circle then planned to bestow Cheetah-like power to Sebastian or something else along those lines, but then Urzkartaga reacted on it's own accord, and so much the better. The question of why Psycho continued to hang around might be because of whatever deal that passed between him and Circe.
I'm pretty sure there are some holes in there, but it's laaate. -_- zzz.