Maybe I wasn't clear. Clark and Lois could've adopted Chris before Jon was born -- and then of course Chris went back to the Phantom Zone. It's not like Chris was around very long -- maybe a month, at most, in "comics time"? And then Lois got pregnant shortly after.
It works provided that LAST SON takes place earlier in Clark and Lois's marriage than it did when it was published.
As for the "New Krypton" story -- honestly, I thought it was a big mess. But a very abbreviated and simplified version could be slotted into "Rebirth" continuity if a writer wanted to put it there. Kandor is taken from Brainiac's ship, restored to normal size, the Kandorians try to live on Earth, things go wrong, they create New Krypton, Zor-El is killed (later to become the new Cyborg Superman!), the Sam Lane/Luthor plan is executed quickly, turning the sun red and blowing up New Krypton, killing all the Kryptonians.
No Mon-El escaping the Phantom Zone, no Superman living on New Krypton, no Zod and Ursa and Non living on New Krypton, no Nightwing and Flamebird, no Lucy Lane as Superwoman, no Sam Lane suicide or even of him being the cause of New Krypton's destruction, no Jim Harper/Guardian subplot, etc.
Just a story that could've taken four issues of SUPERMAN or ACTION COMICS to tell, like it should've been!