My main question was how there could've been a timeline where Comstock even existed if Booker DeWitt died in the revolution, or how likely it was to happen if they reached the point they did at the end anyway.
Then I realized it was a split timeline, not a stable timeloop, and the elderly appearance had other possible explanations. Whoops!
Otherwise though the timeline seems kind of tight for both DeWitt and Comstock, as it seems he gets his city launched in just THREE YEARS after being baptized and gets with Lady Comstock and kills her fairly quickly too. I guess there must've been a decent amount of prep work beforehand, and losing his mind may explain why he was so easy to forgive Lady Comstock of her sins only to go nuts and kill her to protect the secret of Elizabeth.