1. The universe doesn't like its peas being mixed with its porridge. Ultimately what gives Elizabeth her powers is from living in the two universe (some have suggested it's because part of her, her finger, is in on set of universes while she is in the other but I'm not a fan of this, I think the circumstances of who she is and where she's from is the answer and that "peas/porridge" Voxaphone, found in Monument Island, to me, is an agreement that it's simply due to being from another set of universes).
2. Elizabeth opens the point in time where Booker chooses to accept or reject the baptism to find redemption. That moment, and that moment specifically, is a constant. In every single universe in which he is alive, the baptism happens. Basically, this Booker isn't stepping into a new universe, it's like Elizabeth had unlocked the chapter select and picked that checkpoint specifically because in every universe, that checkpoint exists. By murdering that Booker, he becomes a paradox (since that Elizabeth can't exist without Comstock and Booker can't have Elizabeth before the conception occurs). As a result, the only option that can eb picked is to reject the baptism (although that depends on the interpretation of the ending you take from the game, personally I like that one).
3. Nothing. Heads is a constant, it's always heads. Bird or cage is a variable, it's a choice, but it's an ultimately insignificant choice because you don't have the power to change anything significant, an alternate version will, or has, made the other.
4. The archangel is propoganda. Comstock sees the future via the Lutece's tears.
5. Both Booker's are 37 I believe (it's roughly 37, whether it's 37 or 38 I'm unsure). The machine that creates the tears is extremely hazardous to anybody who encounters it. While the game is taking place (in 1912) Comstock is dyinig of cancer and has already been rendered sterile. I believe it was stated in a Voxaphone (I can't remember the specific one) that it increased aging also.
6. Already done:
http://www.abload.de/img/bioshockinfinitetimel19ine.png
Hope that helps.