As for inserting the death Star into ESB, I think the idea has potential and would definitely help to eliminate some of the the WTF? element of the new death star suddenly popping up in ROTJ. It makes sense that straight after the destruction of the DS1, the empire get to work on DS2, making sure this time to close the exhaust ports. It could even be said that this secod death star was already under construction during the time of ANH, but we just never saw it. The question then is how do we put this point across? Changing the title crawl to include this new plot point is one obvious solution (unbeknownst to the rebellion, the empire has commenced work on a second death star...).
Another, more difficult solution is to somehow have one or more characters in the move mention the new death star. Maybe you could have somebody speaking about the new death star via hologram, which might make any lip-sync problems less noticable (blurry image, static, time delay on the voice, etc).