Not sure about Mocha AE, but in Mocha (the full version) you can draw and animate a garbage matte around areas that the tracker needs to ignore. In your case the person walking in front of the screen. That would help Mocha get a better track.
But now that you've got a decent manual track I'm not sure what the problem is.
Placing your video on top of the corner-pin animated background and keying out the blue in the top layer should get you what you want. So what exactly is going wrong there?
Bar3nd
Raamw3rk - digital storytelling and visual effects
