boggler12

The only question I have now is how did Comstock know timeline B booker would engrave 'AD' on his hand? Was it simply him asking what he would do himself, or was male Lutece spying on him before he joined Fem Lutece in Columbia?

The same reason Comstock knew Booker even had a daughter to sell, the same reason he knew he would die and the same reason he knew a 'false shepherd' would come; by looking through a tear to an alternate dimension where it already happened.

Hell, how did he even know Booker would try to get to Columbia in the first place? Its not exactly the easiest place to find and there's too many thing's he'd of had to of guessed to even lead him to that conclusion. Was him going back to Columbia actually intended by Comstock? Did he want to trap him there in hopes to kill him and wrap up the last loose end?

Comstock did not want Booker there. You can see this in the beginning of the game, there's a sticky note on the map that says something to the effect of " Hes coming, stop him" signed -C. This note was intended for the man that was dead in the same room...killed by Lutese to open Booker's passage. Everything Comstock did was to hinder Booker and stop him from rescuing Liz. This may even explain the creation of Songbird. Old Elizabeth tells Booker that Songbird stops him from rescuing her in every reality, every time; thus enabling the destruction of New York and the fruition of Comstocks' plans. It's possible Comstock also saw this and created the very thing that he saw continually stop Booker. It's a bit of a stretch though.

One thing I didn't understand is what was different about this reality that allowed Old Elizabeth to resist her conditioning and bring Booker to 1984? It was this action that set this reality apart and allowed them to control Songbird and destroy the Siphon, unlocking Elizabeth's powers. This allowed her to see how every reality was following the same two paths that eventually converged into one; the destruction of New York. It also allowed her to see what was necessary to stop it. Obviously this had not happened in any other reality, so what made her able to do it in this one?