You can't have it both ways. Either Itachi was coldly, rationally and unemotionally considering all options, or he was trying to save his brother at all costs (even sacrificing his own parents).Being Itachi and his analytically self, I'm sure he considered it. But, I think he realized that he was helpless to try and stop his clan from thinking about rebellion. He had been given 2 options by Danzo, and he chose the one option that would allow the Uchiha clan to live on in the form of Sasuke.
Remember that the reason Itachi did all of this was because of Sasuke. He loves his younger brother quite alot. I doubt he trusted Danzo, but maybe he did, just enough to believe his word the Sasuke would be safe. As for Tobi, he placed conditions upon their collaboration. He would aid him in exacting revenge on the Uchiha (whose motive for revenge is unknown, to this point), but only if he never attacks Konoha again, and stays away from Sasuke. The bond between brothers is quite strong.
In the first case, why didn't Itachi choose to cooperate with the Third? More than that: he could have hidden Sasuke or given him to the Third for protection, if he was just trying to choose the most rational option. Then, he could have tried to do whatever was possible to stop the rebellion. This might have ended in a war anyway, but at least he would be doing his best to protect both his family and the village.
In the second case, if Itachi was just reacting out of fear for his brother and for the village, his actions make a little more sense. A terrified person does irrational things. However: Itachi was already a spy (double agent), and he was controlled enough to continue this mission succesfully. This is more consistent with being a fanatical and blind follower of Danzo.
If you believe that Itachi was rational and controlled, than the only thing that fits his actions is either a direct attack on him, or a fanatical loyalty to Danzo/Root.