So the lead dev has a private key which lets him control all the MasterNodes? Why does he need to control all MasterNodes? In the future, will he be in control or is this just temporary and if so how long will this continue?
No, he does not. There is a private key for engaging the spork but that's it. This allows us to prevent forks for the time being while bugs are getting troubleshooted on mainnet when new features get rolled out.