I'd say your new attempt is quite an improvement, and probably not possible to get clean audio in a silent scene.

As to the amplitude change, I'm not sure it matters in any case. I used the Amplify effect in Audacity to make the volume of both tracks similar. The thinking behind it was carried over from when I was trying to cancel out audio by inverting the waveform of the copied audio track, and to successfully cancel the two waveforms they needed to be of equal height. Again, not sure that it matters when using Knockout, but I did the same thing to the Leia hologram speech and it seemed to improve the quality.