Nicely put, but Comstock isn't only in one reality, we pass through at least two or three where he exists. How does our Booker drowning prevent the creation of other Comstocks when we know he is "gambling alcho" Booker? The Booker that Elizabeth drowns at the baptism isn't the Booker who becomes Comstock, is he...?
It all, ultimately, is due to the paradox. Before the events of the game, there are infinite Comstocks. There are infinite Bookers. There are infinite everybodys and everythings. All of these are subsets of other subsets of other subsets of infinite subsets. In every single universe where Booker does not die in Wounded Knee, he MUST attend the baptism. This is a fixed point in every single reality. Since every Booker is forcibly drowned by Elizabeth at the baptism (it has to be before a choice is made at the baptism because, if it is after, some Bookers may be baptised later on and then become Comstock trapping them in the 'loop' again), every single Booker dies, in every reality. What this basically means is that, in every single reality no matter what, Booker dies. However, Elizabeth's existence can only occur if Comstock exists (which means Booker accepted the baptism). Now, there were an infinite set of Comstocks, there were some where Comstock founded Columbia and it was paradise, some where he never used the machine, some where he never stole Anna and so on, but none of this matters because, in one universe, Comstock existing made every single existence of Comstock a paradox (because Elizabeth murdered every Booker before any decision was made). The only possible set of realities that can exist, without the paradox, are universes where Elizabeth doesn't exist and thus, universes where Comstock cannot exist, making the rejection a constant (there are still an infinite number of Bookers that reject the baptism though).
EDIT:
I probably need to look into it more, but I imagine (or like to imagine anyway) that these constants are more for if certain events are to occur: for Bioshock Infinite to happen (and perhaps by association all events Elizabeth can look into) Booker MUST go to baptism. Any other possibility (never going to the baptism, never going into the military, being born female, not being born at all) would not have resulted in Bioshock Infinite or led to it. As such, she eliminated the constant necessary for her existence as she is, and all that's left are the other possibilities that don't involve going to baptism out of guilt from the war.
Of course I guess just eliminating the possibility he COULD accept baptism is enough, but there's that implication of avoiding the baptism situation at all.
I will need to think further on this but my immediate reaction is that the constants are things that always happen in universes with a specific set of variables and the constants are precisely that, constants, things that must happen. The reasoning for this is related to the introduction of the constant and variable segment (with regard to the lighthouse) where emphasis is placed on always (which is after Elizabeth's full set of power is granted). 'There are a million million of worlds, all different, all similar. Constants and variables. There is always a lighthouse, there's always a man, there's always a city. I can see them through the doors' (roughly) to me anyway, fixes the ideas that the constants always happen. So, in every universe that exists, there is a man, a lighthouse, a city. Similarly, in every universe where Booker survived Wounded Knee (and thus, in every universe where he went through these events) the constant of the baptism exists. I'm not sure if that really addresses the question of the specificities of the constants and variables though.
All those possibilities already exist in all the infinite universes with their infinite choices. 