Why does miss havisham manipulate and mislead pip into thinking she is his secret benefactor? What could she get from his behavior?

It actually was never in Miss Havisham's plan that Pip believe her to be his benefactor. Her plan was for Pip to fall in love with Estella so that she could break his heart. She manipulated and misled him into belleving he had a chance for Estella to fall in love with him, and that possibly they might have a future. As we all know, this was not her intent. Her desire wasn't to make a gentleman of Pip, but rather to play him for a fool in the same way that she had been played for a fool.