Wait a minute. (Implications of Burial at Sea) [Spoilers]

Below are spoilers for the ending of Infinite. Fair warning.

So after I beat Bioshock Infinite my buddy and had a long conversation about it. I inquired about the whole Rapture thing because I have never played the original Bioshock (Heathen, I know, but the sound design in that game terrifies me, I can't play for longer than like 30 minutes at a time). I asked him if it meant anything or if it was just a fanservice nod to those who had played both games (I'm going with Yahtzee's opinion that Bioshock 2 doesn't count).

Anyway, his explanation was that popular theory went that Comstock/Booker were alternate world mirrors of Ryan/protagonist (With Elizabeth being split into multiple Little Sisters/Songbird being Big Daddy's). I tried to call shenanigans on that but he said the evidence was in the fact that at the end of Inifinite, when you walk through Rapture you operate a Bathy Sphere and those could only be operated by Ryan/someone with is DNA.

Well I had nothing to refute that particular point of logic, but I always felt like that was just an oversight on Irrational's part.

So now this trailer for Burial at Sea comes out. Am I totally wrong or does the implications of the trailer blow this particular theory out of the water? If Booker and Elizabeth exist as individuals in Rapture then they can't be the protagonist/Little Sisters. Which means that Bathy Sphere thing was just an oversight (Or Booker is a cousin/brother/uncle/nephew of Ryan?)

Which brings me back to my original question for my buddy. What was the point of the Rapture walk through at the end of Infinite other than Fan Service? Yes, Rapture exists in an alternate universe from Infinite and they are all connected somehow (There is always a lighthouse) but was walking through Rapture really necessary, or was it simply a shortcut to get this multi-verse idea across?