So, after a lot of to-ing and fro-ing and reading a lo-o-ot of this thread, here's where I'm at right now.
In the end baptism scene, before Liz's reality disappears, Liz looks towards the door and the camera pans up to it. This must mean something. It signifies that there is something more than simply the Anna/Liz timeline disappearing from what is, was and will be. It's like she's thinking to herself "My work is done here, but there are other realities that will keep going on through that door". The post-credits scene backs this up and I don't think it was just tacked on. Perhaps I'm underestimating the power of people wanting a happy ending, but I'm not sure a single sale of the game would be effected by the inclusion or not of that scene. Personally I'd think the game would have wrapped up well enough without it, but I like those kinda endings.
So, I'm concluding that the post credit scene is 'canon'. This tells me that there is a timeline after the baptism for Booker and Anna.
This just leaves me with "how"? It's clear that Booker was killed before he made the choice. "Before the choice is made" and "No, I'm both" make that hard to get around. The fact that he doesn't struggle much indicates that he accepts his fate and it's probably the only real shot at redemption he has. Either that or the writing was badly ambiguous - I'm assuming it's not for the time being.
This kinda leaves the theory that in any event where booker said yes to baptism, that would automatically create instances where the events of the game take place - Comstock combines with Lutece, Columbia is created, Liz is bought and Bookers are brought through tears as puppets for the Luteces to fix it. So then, in any instance of Booker saying "yes" to baptism, it would automatically flow from that the "time lord" instances of Liz who would stop it from ever happening. This would automatically close off any possibility of the events ever happening.
In effect, what we played in the game was in fact just a closed set of possibilities of events that get closed off even before they begin. As a few people have said, a variable becomes a constant.
The fact that he sounds surprised in the post credit scene when he calls out Anna's name indicates that (a) he has sold Anna but she's somehow back, (b) he's just wishing she was back but she isn't or (c) he somehow has memories of his other timeline experiences in Columbia and is worried that Anna isn't there, even though she is safe and sound (depressed, gambling, alcoholic father aside). I'm going to ponder that further but I'm leaning towards C. C would give Booker the "A Christmas Carol" foresight to turn his and Anna's life around - probably the thing that Liz would have hoped for in the end.