I personally haven't seen the loading screen for nuketown, but from what I hear, that's generally agreed upon and the post card from green run was slipped in the side by it, stating that the great leap forward comes after green run and of course after nuketown. And now that I think about it, the green run looks like it was just slid into there, so these levels may even be out of order, or just slid into there because they were on the bus, it fits, and is just there to show they made a stop on the way to provinince 22.
Just an observation on the Nuketown loading screen - the figure in the bottom left. There seems to be a slight glow around him and you can see it's Skeletal Structure - Could this be the Avogardo? I know it's contained in the device in the power room on Green Run, but I saw it an just thought I could be connected. The glowing represents it's electricity perhaps?
EDIT - - ALSO, the outline of the person on the left reminds me of 1 of 2 people:
1) The outline of Mason on the artwork of the game / title screens
2) The outline of Samuel when he's stood in the opening Cinematic for Die Rise.
However, to slightly agree with the theory previously mentioned earlier about it being Richtofen on the right, could it be the outline of Maxis? Do we even know what he looks like?