So, the Letuces' basically have an about-face moment when they realize how out of hand Comstock has gotten?
Go crazy and attack this theory.
The divide by 0. Elizabeth/Anna killing Comstock/Booker at the Baptism eliminates both possibilities. The one when he has the child and the one when creates Columbia. Thus eliminating their existence.
Technically all that this means is that the Booker's who made the choice of even going to the baptism event were the ones who were killed. If Booker dying represents the death of the baptism choice, then it almost seems like the universe corrects itself to skip the event itself, leaving other universe's Booker's still alive.
Plus, in the post-credit scene, Booker is apparently agitated about Anna being missing, or alternatively, suddenly returning, it implies that he has some awareness/hazy recollection of either the events of infinite or at least the loss of his kid, which seems to imply to me that Liz pulled some cosmic strings to give at least one Booker a semi-happy ending.
There's always a lighthouse and there is always a guy, but does that guy necessarily have to be Booker? I thought that is what they were alluding to by showing us Rapture. In that scenario there was a lighthouse and there was a guy. Then again, there was no girl in that; unless we count the little sisters.
You may be right though. We're dealing with Infinite possibilities and it's hard to fathom that in all of them Booker just happens to end up at the baptism. But that could be the fun cosmic joke. Maybe it's some sort of fixed point in the multiverse.
Interestingly enough, Bioshock 2 has Eleanor, which is a much more centralized character than any female in Bioshock besides the random little sisters, but they did play a fairly important role in determining Jack's character, since he adopts them in the 'good' ending.
Also, while I hate the 'unreliable narrator' device as is heavily used here by Booker's memory lapse (though it is explained in part by the Letuce's in the universe travel aspect), I did chuckle when I saw the opening message about memories being unreliable.