I believe that God designed us with the ability choose to do what is right, and at the same time he knew that we would choose instead to sin. Otherwise, if we were designed with no ability to choose to do right, we would not be responsible for doing wrong—God would be. The righteousness God intended for us was a righteousness from Christ, not of ourselves. In that this works, God’s plan is effective, not impotent.

I did not mean to imply that God’s plan and his purpose are not in harmony, merely that they are distinct. The plan gets history to God’s purpose—it is not itself the culmination of that purpose. (I’m in agreement with the way you use plan=method above.)

To summarize, as I understand it God’s plan always was for those people with whom he’d dwell in eternal glory to be there “in Christ” and not on the basis of their own righteousness. And this plan is going to work (Rom. 8:28ff).

I agree with your final point—that the eternally lost will have chosen to be so lost.