There is no way--no way ever--that Zhuge Liang would ever usurp the throne. He was a film believer in patrilineal succession (evidenced by his distaste for Liu Feng's position), and would never deign to replace the Liu bloodline, which he saw as an essential part of the leader of the empire, no matter how incompetant the person. He would have acted as an advisor, to help show Liu Shan how he would be expected to act, etc., but wouldn't rule directly.
In addition, there is no way that someone like Huang Hao would have come to power if Zhuge Liang was around. Zhuge Liang would have had a lot more swing at court than Jiang Wei did, and he proved that he was more than willing to use his authority to dispatch people who failed the empire in some way (even Ma Su).
"If I had to do my life over, I would change every single thing I have done."
— Raymond Douglas Davies, 1967