Editors don't have much power anyway. The author can take his his advice or not. Only morons do everything the editor says.
Not sure about that, Editors can pretty much reject scripts if they don't feel like the manga is going in the right direction, specially for the newer mangaka, it's why we see so many similar plots and tropes getting used by manga published by the same magazine.
I think Bakuman gave some insight in how editors can push mangaka in the wrong direction if they have certain preferences.
It's a two way street, if the mangaka doesn't listen to their editor and the editor has genuine concerns about the direction, the manga could be pulled from the magazine, editors are basically the mouthpiece for the magazine and what it represents, if the editor says no to any of the content, it's basically the magazine saying no. So it really depends on how well the two can work together.