The way I see it, the alleged over 40 000 pound weight gain comes effectively from 40 000 pound difference from cruise fuel burn. Correct?
I looked for the tail engine air inlet on the video and could not see it. Would not a big APU with 16 t thrust require a substantial air inlet? It is over half the total thrust of MD-11 or Tristar tail engine, remember!
I accept that you plan to make the wall thicker in real life. But what precisely is the advantage of narrowing a 747 like double decker main deck? The fitting of double bubble is uncomfortable as it is, and narrowing the main deck will make matters worse.