Is it one reality where there was a Booker that didn't get baptized, or many?
The way I interpreted it was what others have said. The split happened with him walking away, and that version has the baby. I think in those versions, that's where the male Lutece comes from, while the female one comes from the other line. She experiments with the idea of the tears, then sees one where a male version of her exists. Maybe by digging around that one and bringing him over, that's when the Comstock sees the other Booker that had a kid. Seeing this, probably thinks it's a holy hand showing him his own child, so he has to have her. Gets kid, Letuce's start regretting what they've done, killed by own machine, now "trapped" in the inner workings, see the only way to atone for what they've done is to also get Booker to atone for what he did. Only in that fashion can their wrong be righted.
Guessing that what we see post credits is Booker, as in the player one that has all of his memories of what he's gone through, back in his apartment, and finds the crib in next room as we quickly cut to back. Obviously this was shown because the God Liz is still alive, and decided to throw her "dad" a bone, as he made the right choice in killing Comstock before any harm was done to her. The bird or the cage: To be free, or to be jailed. Would love if you only get this scene if you chose the bird necklace near the beginning, but probably shows no matter what you chose.
