@Dronz3r I'm just guessing here, but I think the answer to your first question is that there is, in fact, a scenario where Booker did end up in a Comstock-free world. That's just not OUR Booker. We had to find the way to see that Comstock-free world and our path to that was through the drowning. After that, we got to see what that world looked like at the end of the credits with Booker and Anna together. At least that's what I want to believe.

I think @maxmac and @saint311 answered part two very well.

@elitesoldier117 The optimist in me agrees with you. The last scene where we see him go into her room is the Booker/Anna that lives happily ever after.