Interesting side note, Did anyone else pick up on when powergirl said that Superman prposes to lis on top of the Daily Star building? this doesn't match what happened in the tale Superman takes a bride. They got maried while Clark forgot that he was Superman. So point to poner, is this truely the E-2 couple we've been hoping for?
Well, part of why I consider Johns overrated is his sloppy, shoddy research. You have a case in point above.
I suppose we might cut him some slack if we remember that Clark Kent married Lois Lane first, but when they rediscovered his secret identity, Superman decided to wed Lois as well using Kryptonian customs in his Secret Citadel. While it wasn't shown in the original wedding issue, a scene might be shoehorned in between Superman with Lois in their Metropolis home and the flight to the Citadel that he got the coal from Zriff (or was that supposed to be Mxy's 'Zrff'?), compressed it into a diamond, then proposed atop the Daily Star building.
But that's a heckuva lotta 'supposing' just to wipe out Johns' sloppy research.
I don't think Superman originally had any kind of ulterior motives in coming to the mainstream DCU. He came precisely why he said he did to save it from itself. Lois was dying from normal aging in the other realm and she didn't get so bad until after he brought her back to the real DCU. Since she was otherwise fine and even better off in the other realm, he cannot be faulted for an ulterior motive here.
Now, he does have much happier memories of his Earth-2 history which conflict with the abyssmal hell of the Iron Age DCU he's been observing. Of course, he'd think his old Earth would be an improvement over the one he's just witnessed in comparison. But I didn't get the sense that he specifically intended to retrofit E-2 into the mainstream DCU as the replacement. He did say their old Earth was forgotten and this dark world needed to be just as forgotten.
"
This corrupted and darkened Earth must be forgotten as
ours was ... so that the
right Earth can return."
-- E-2 Superman to Power Girl, Infinite Crisis #2, final page, panel 4.