One thing I didn't see explained in the thread (maybe I just missed it). At the end, Elizabeth says she can't open a door to a certain lighthouse, but then all of a sudden she has the key in her hand and says it's always been there, somehow. Not sure I followed that, is it supposed to be a metaphor or something?
Also, theoretically, wouldn't it be possible for more than two Bookers to exist in the same Colombia, if the Luteces were to bring one back through the tears, and then go and get another? If Booker can meet Comstock (which essentially is a different version of himself), why wouldn't he be able to meet other Bookers as well?