What "imposter" did Pio encounter in chapter 58? What is Pips plan upon returning to the forge? Why doesn't his plan succeed?

I'm not sure what you mean by imposter, but in chapter Pip wants to see Biddy, and he is determined to see that her marriage is one of convenience, that one shouldn't have expectations, and that they should just settle. He learns this isn't true when he comes face to face with Joe and Biddy. Pip was still adhering to societal concepts of what is happiness (this one taking place in a simple village with a simple wife) instead of seeing people for who they really are and appreciating the relationship beyond its societal label. Thus, his misconception is wrong.