Ok, so now having twelve hours or so to digest the ending, here's what I think happened with the post credit sequence...
Troy Baker and Lizzy McLizbeth played guitar and sang.
... Also...
I think Comstock was "killed in his crib," but not Booker. I think Booker was taken to the baptism twice for a very specific reason pertaining to the one binary choice which determines Comstock's existence. It was the place where Comstock was born, but only in one branch out of two. So the first visit was the Booker path, in which he lived, and the second was the Comstock branch, where he was killed. Thus, Booker realities continue, but not Comstock.
I don't need to spoiler tag anything in here, right?