How does Lady Macbeth influence the plot development?

Lady Macbeth is hugely influential in the early stages of the plot's development. As the the driving force behind the murder of Duncan, she's arguably the most important character in the first two acts. She dominates events on stage, constantly cajoling her reluctant husband to act out his destiny and claim the throne for himself. She's also responsible for planning the logistics of Duncan's killing and its subsequent cover-up down to the very last detail.
Once Macbeth becomes king, however, Lady Macbeth gradually fades from view. The power relationship she enjoyed with her husband changes completely. Macbeth becomes obsessed with fulfilling the terms of the witches' prophecy. He no longer feels that he needs to have Lady Macbeth by his side; he can rely upon the forces of darkness instead. That's why he embarks upon a series of senseless murders, which would never have been carried out had Lady Macbeth still been there to guide him.
After Duncan's murder, the plot is largely driven by the witches' prophecy and Macbeth's obsession with it. In the meantime, Lady Macbeth is reduced to a peripheral figure, gradually going insane as the bodies start piling up and yet more blood is shed. Whereas before she was the main instigator of the action, now she is little more than the plaything of events. Long before her death, she has been reduced to playing a bit part in a huge cosmic drama in which the forces of good and evil are engaged in a titanic struggle.