(Sorry for the bother and if this is annoying to deal with but) why do the humans consider themselves to have a one-way biological relationship (alpha kids --> beta kids) rather than the trolls do with dancestorship, seeing the other as both their ancestor and descendant (beforus kids <---> alternia kids)? Aren't they ectobiologically the same, despite the trolls' different manner of breeding?

Yes, this is a cultural matter. In troll culture they don’t have parents, there is no direct line of biology for them to follow. It also has to do with troll genetics, while it’s easily seen that each beta kid is made from two of the alpha kids with the trolls it’s more difficult.  We don’t know the specifics of the trolls’ ectobiology but it’s entirely likely that each one of Meenah’s group of trolls could have been made through a mixture of three trolls, or even a mixture of all the trolls with the one that contributed the most genetics sharing the symbol. There are other theories about troll ectobioloy out there but overall troll biology is much more complicated then human biology.