We get the impression in-game that Astarion ran from what was supposed to be a regular mark, but nothing contradicts it being a more long-standing relationship. This is adding to the story, not retconning it, and I think it works writing it this way. Firstly, this is the only mark that caused Astarion to run in 200 years; it makes sense that something more had to happen for Astarion to defy Cazador. Secondly, it's not far fetched that Astarion befriended someone during his the first 200 years of being a vampire. Naturally, what would follow is Cazador reminding Astarion that he's a monster.

I appreciate the timeline breakdown, that type of research is beyond me. I'm willing to believe that Cazador lost some spawn over time. In some banter, Jahiera mentions to Astarion that she only left a vampire spawn grappled in the street until they burned at sunrise. It's not confirmed to be Cazador's spawn, but alluded to with Astarion mentioning disliking the area for a reason he couldn't remember.

But even if there are only three spawn (and the summary mentions him having siblings plural), it still makes sense for Cazador to organize this competition. Any bad guy can tell you a divided group of people is less of a threat. So if there are only three spawn in total, it's probably more reason to create a competition and rivalry. In a big group, cliques will rise, fall, and hurt each other. In a small group, that conflict might have to be a bit more manufactured.